diff --git a/etc/aiscript.api.md b/etc/aiscript.api.md
index b35a710b..6813e93f 100644
--- a/etc/aiscript.api.md
+++ b/etc/aiscript.api.md
@@ -177,14 +177,6 @@ declare namespace Ast {
 }
 export { Ast }
 
-// @public (undocumented)
-type Attr_2 = {
-    attr?: {
-        name: string;
-        value: Value;
-    }[];
-};
-
 // @public (undocumented)
 type Attribute = NodeBase & {
     type: 'attr';
@@ -855,8 +847,6 @@ export class Scope {
     assign(name: string, val: Value): void;
     // (undocumented)
     createChildNamespaceScope(nsName: string, states?: Map<string, Variable>, name?: Scope['name']): Scope;
-    // Warning: (ae-forgotten-export) The symbol "Variable" needs to be exported by the entry point index.d.ts
-    //
     // (undocumented)
     createChildScope(states?: Map<string, Variable>, name?: Scope['name']): Scope;
     exists(name: string): boolean;
@@ -968,7 +958,7 @@ function valToJs(val: Value): any;
 function valToString(val: Value, simple?: boolean): string;
 
 // @public (undocumented)
-type Value = (VNull | VBool | VNum | VStr | VArr | VObj | VFn | VReturn | VBreak | VContinue | VError) & Attr_2;
+type Value = VNull | VBool | VNum | VStr | VArr | VObj | VFn | VReturn | VBreak | VContinue | VError;
 
 declare namespace values {
     export {
@@ -983,7 +973,6 @@ declare namespace values {
         VBreak,
         VContinue,
         VError,
-        Attr_2 as Attr,
         Value,
         NULL,
         TRUE,
@@ -1004,6 +993,30 @@ declare namespace values {
 }
 export { values }
 
+// @public (undocumented)
+export type Variable = ({
+    isMutable: false;
+    readonly value: Value;
+} | {
+    isMutable: true;
+    value: Value;
+}) & {
+    type?: Type;
+    attrs?: Attr_2[];
+};
+
+// @public (undocumented)
+export const Variable: {
+    mut(value: Value, opts?: {
+        type?: Type;
+        attrs?: Attr_2[];
+    }): Variable;
+    const(value: Value, opts?: {
+        type?: Type;
+        attrs?: Attr_2[];
+    }): Variable;
+};
+
 // @public (undocumented)
 type VArr = {
     type: 'arr';
@@ -1078,6 +1091,11 @@ type VStr = {
     value: string;
 };
 
+// Warnings were encountered during analysis:
+//
+// src/interpreter/variable.ts:14:2 - (ae-forgotten-export) The symbol "Type" needs to be exported by the entry point index.d.ts
+// src/interpreter/variable.ts:15:2 - (ae-forgotten-export) The symbol "Attr_2" needs to be exported by the entry point index.d.ts
+
 // (No @packageDocumentation comment for this package)
 
 ```
diff --git a/src/index.ts b/src/index.ts
index a2420552..3aa0df04 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -6,6 +6,7 @@ import { Interpreter } from './interpreter/index.js';
 import { Scope } from './interpreter/scope.js';
 import * as utils from './interpreter/util.js';
 import * as values from './interpreter/value.js';
+import { Variable } from './interpreter/variable.js';
 import { Parser, ParserPlugin, PluginType } from './parser/index.js';
 import * as Cst from './parser/node.js';
 import * as errors from './error.js';
@@ -15,6 +16,7 @@ export { Interpreter };
 export { Scope };
 export { utils };
 export { values };
+export { Variable };
 export { Parser };
 export { ParserPlugin };
 export { PluginType };
diff --git a/src/interpreter/index.ts b/src/interpreter/index.ts
index 23df41ea..f2e03950 100644
--- a/src/interpreter/index.ts
+++ b/src/interpreter/index.ts
@@ -4,6 +4,7 @@
 
 import { autobind } from '../utils/mini-autobind.js';
 import { AiScriptError, NonAiScriptError, AiScriptIndexOutOfRangeError, AiScriptRuntimeError } from '../error.js';
+import { getTypeBySource } from '../type.js';
 import { Scope } from './scope.js';
 import { std } from './lib/std.js';
 import { assertNumber, assertString, assertFunction, assertBoolean, assertObject, assertArray, eq, isObject, isArray, expectAny, reprValue } from './util.js';
@@ -370,19 +371,23 @@ export class Interpreter {
 
 			case 'def': {
 				const value = await this._eval(node.expr, scope);
+				let attrs: Variable['attrs'] = undefined;
 				if (node.attr.length > 0) {
-					const attrs: Value['attr'] = [];
+					attrs = [];
 					for (const nAttr of node.attr) {
 						attrs.push({
 							name: nAttr.name,
 							value: await this._eval(nAttr.value, scope),
 						});
 					}
-					value.attr = attrs;
 				}
+				let type = undefined;
+				if (node.varType) type = getTypeBySource(node.varType);
 				scope.add(node.name, {
 					isMutable: node.mut,
-					value: value,
+					value,
+					type,
+					attrs,
 				});
 				return NULL;
 			}
diff --git a/src/interpreter/value.ts b/src/interpreter/value.ts
index 0933bc18..a43301ac 100644
--- a/src/interpreter/value.ts
+++ b/src/interpreter/value.ts
@@ -67,14 +67,7 @@ export type VError = {
 	info?: Value;
 };
 
-export type Attr = {
-	attr?: {
-		name: string;
-		value: Value;
-	}[];
-};
-
-export type Value = (VNull | VBool | VNum | VStr | VArr | VObj | VFn | VReturn | VBreak | VContinue | VError) & Attr;
+export type Value = VNull | VBool | VNum | VStr | VArr | VObj | VFn | VReturn | VBreak | VContinue | VError;
 
 export const NULL = {
 	type: 'null' as const,
diff --git a/src/interpreter/variable.ts b/src/interpreter/variable.ts
index 30252b5d..de2d1e77 100644
--- a/src/interpreter/variable.ts
+++ b/src/interpreter/variable.ts
@@ -1,6 +1,7 @@
 import type { Value } from './value.js';
+import type { Type } from '../type.js';
 
-export type Variable =
+export type Variable = (
   | {
     isMutable: false
     readonly value: Value
@@ -9,18 +10,28 @@ export type Variable =
     isMutable: true
     value: Value
   }
+)	& {
+	type?: Type
+	attrs?: Attr[];
+};
+export type Attr = {
+	name: string;
+	value: Value;
+};
 
 export const Variable = {
-	mut(value: Value): Variable {
+	mut(value: Value, opts?: { type?: Type, attrs?: Attr[] }): Variable {
 		return {
 			isMutable: true,
 			value,
+			...opts,
 		};
 	},
-	const(value: Value): Variable {
+	const(value: Value, opts?: { type?: Type, attrs?: Attr[] }): Variable {
 		return {
 			isMutable: false,
 			value,
+			...opts,
 		};
 	},
 };
diff --git a/src/type.ts b/src/type.ts
index 5e93acf0..4def411b 100644
--- a/src/type.ts
+++ b/src/type.ts
@@ -1,13 +1,27 @@
-import { AiScriptSyntaxError } from './error.js';
+import { AiScriptTypeError } from './error.js';
 import type * as Ast from './node.js';
 
-// Type (Semantic analyzed)
+// Types (Semantically analyzed)
+
+export type Type = TSimple | TGeneric | TFn;
 
 export type TSimple<N extends string = string> = {
 	type: 'simple';
 	name: N;
 }
 
+export type TGeneric<N extends string = string> = {
+	type: 'generic';
+	name: N;
+	inners: Type[];
+}
+
+export type TFn = {
+	type: 'fn';
+	args: Type[];
+	result: Type;
+};
+
 export function T_SIMPLE<T extends string>(name: T): TSimple<T> {
 	return {
 		type: 'simple',
@@ -15,16 +29,6 @@ export function T_SIMPLE<T extends string>(name: T): TSimple<T> {
 	};
 }
 
-export function isAny(x: Type): x is TSimple<'any'> {
-	return x.type === 'simple' && x.name === 'any';
-}
-
-export type TGeneric<N extends string = string> = {
-	type: 'generic';
-	name: N;
-	inners: Type[];
-}
-
 export function T_GENERIC<N extends string>(name: N, inners: Type[]): TGeneric<N> {
 	return {
 		type: 'generic',
@@ -33,12 +37,6 @@ export function T_GENERIC<N extends string>(name: N, inners: Type[]): TGeneric<N
 	};
 }
 
-export type TFn = {
-	type: 'fn';
-	args: Type[];
-	result: Type;
-};
-
 export function T_FN(args: Type[], result: Type): TFn {
 	return {
 		type: 'fn',
@@ -47,51 +45,51 @@ export function T_FN(args: Type[], result: Type): TFn {
 	};
 }
 
-export type Type = TSimple | TGeneric | TFn;
-
-function assertTSimple(t: Type): asserts t is TSimple { if (t.type !== 'simple') { throw new TypeError('assertTSimple failed.'); } }
-function assertTGeneric(t: Type): asserts t is TGeneric { if (t.type !== 'generic') { throw new TypeError('assertTGeneric failed.'); } }
-function assertTFn(t: Type): asserts t is TFn { if (t.type !== 'fn') { throw new TypeError('assertTFn failed.'); } }
+function isTSimple(x: Type): x is TSimple { return x.type === 'simple'; }
+function isTGeneric(x: Type): x is TGeneric { return x.type === 'generic'; }
+function isTFn(x: Type): x is TFn { return x.type === 'fn'; }
+function isAny(x: Type): x is TSimple<'any'> { return x.type === 'simple' && x.name === 'any'; }
 
 // Utility
 
+/**
+ * Checks value of type b is assignable to type a,
+ * or in other words type a is superset of type b.
+ */
 export function isCompatibleType(a: Type, b: Type): boolean {
-	if (isAny(a) || isAny(b)) return true;
-	if (a.type !== b.type) return false;
+	if (isAny(a)) return true;
+	if (isAny(b)) return false;
 
+	// isTSimple系のif文で分岐すると網羅性チェックができない？
 	switch (a.type) {
-		case 'simple': {
-			assertTSimple(b); // NOTE: TypeGuardが効かない
-			if (a.name !== b.name) return false;
-			break;
-		}
-		case 'generic': {
-			assertTGeneric(b); // NOTE: TypeGuardが効かない
-			// name
-			if (a.name !== b.name) return false;
-			// inners
-			if (a.inners.length !== b.inners.length) return false;
-			for (let i = 0; i < a.inners.length; i++) {
+		case 'simple' :
+			if (!isTSimple(b)) return false;
+			if (!(a.name === b.name)) return false;
+			return true;
+
+		case 'generic' :
+			if (!isTGeneric(b)) return false;
+			if (!(a.name === b.name)) return false;
+			if (!(a.inners.length !== b.inners.length)) return false;
+			for (const i in a.inners) {
 				if (!isCompatibleType(a.inners[i]!, b.inners[i]!)) return false;
 			}
-			break;
-		}
-		case 'fn': {
-			assertTFn(b); // NOTE: TypeGuardが効かない
-			// fn result
+			return true;
+
+		case 'fn' :
+			if (!isTFn(b)) return false;
 			if (!isCompatibleType(a.result, b.result)) return false;
-			// fn args
-			if (a.args.length !== b.args.length) return false;
-			for (let i = 0; i < a.args.length; i++) {
+			if (!(a.args.length !== b.args.length)) return false;
+			for (const i in a.args) {
 				if (!isCompatibleType(a.args[i]!, b.args[i]!)) return false;
 			}
-			break;
-		}
+			return true;
 	}
-
-	return true;
 }
 
+/**
+ * Type to string representation
+ */
 export function getTypeName(type: Type): string {
 	switch (type.type) {
 		case 'simple': {
@@ -151,7 +149,7 @@ export function getTypeBySource(typeSource: Ast.TypeSource): Type {
 				return T_GENERIC(typeSource.name, [innerType]);
 			}
 		}
-		throw new AiScriptSyntaxError(`Unknown type: '${getTypeNameBySource(typeSource)}'`);
+		throw new AiScriptTypeError(`Unknown type: '${getTypeNameBySource(typeSource)}'`);
 	} else {
 		const argTypes = typeSource.args.map(arg => getTypeBySource(arg));
 		return T_FN(argTypes, getTypeBySource(typeSource.result));