diff --git a/docs/syntax.md b/docs/syntax.md
index 9962bb74..4d491f1d 100644
--- a/docs/syntax.md
+++ b/docs/syntax.md
@@ -71,7 +71,29 @@ let add2 = @(x, y) {
 ) {
 	x + y
 }
-@add5(x,y){x+y} // ワンライナー
+// 省略可能引数
+@func1(a, b?) {
+	<: a
+	<: b // 省略されるとnullになる
+}
+func1('hoge') // 'hoge' null
+// 初期値を設定された引数（省略可能引数と組み合わせて使用可能）
+@func2(a, b?, c = 'piyo', d?) {
+	<: a
+	<: b
+	<: c
+	<: d
+}
+func2('hoge', 'fuga') // 'hoge' 'fuga' 'piyo' null
+// 初期値には変数を使用可能（値は宣言時点で固定）
+var v = 'hoge'
+@func3(a = v) {
+	<: a
+}
+v = 'fuga'
+func3() // 'hoge'
+// ワンライナー
+@func4(a,b?,c=1){<:a;<:b;<:c}
 ```
 ```js
 // match等の予約語は関数名として使用できない
@@ -87,6 +109,8 @@ var func = null
 @func() { // Runtime Error
   'hoge'
 }
+// 省略可能引数構文と初期値構文は併用できない
+@func(a? = 1) {} // Syntax Error
 ```
 
 ### 代入
diff --git a/etc/aiscript.api.md b/etc/aiscript.api.md
index caf7f7e2..9423f773 100644
--- a/etc/aiscript.api.md
+++ b/etc/aiscript.api.md
@@ -279,6 +279,8 @@ type Fn = NodeBase & {
     type: 'fn';
     args: {
         name: string;
+        optional: boolean;
+        default?: Expression;
         argType?: TypeSource;
     }[];
     retType?: TypeSource;
@@ -625,6 +627,7 @@ declare namespace values {
         VObj,
         VFn,
         VUserFn,
+        VFnArg,
         VNativeFn,
         VReturn,
         VBreak,
@@ -685,6 +688,13 @@ type VError = {
 // @public (undocumented)
 type VFn = VUserFn | VNativeFn;
 
+// @public (undocumented)
+type VFnArg = {
+    name: string;
+    type?: Type;
+    default?: Value;
+};
+
 // @public
 type VNativeFn = VFnBase & {
     native: (args: (Value | undefined)[], opts: {
@@ -729,10 +739,15 @@ type VStr = {
 // @public (undocumented)
 type VUserFn = VFnBase & {
     native?: undefined;
+    args: VFnArg[];
     statements: Node_2[];
     scope: Scope;
 };
 
+// Warnings were encountered during analysis:
+//
+// src/interpreter/value.ts:46:2 - (ae-forgotten-export) The symbol "Type" needs to be exported by the entry point index.d.ts
+
 // (No @packageDocumentation comment for this package)
 
 ```
diff --git a/src/interpreter/index.ts b/src/interpreter/index.ts
index 3401b6d5..750c108c 100644
--- a/src/interpreter/index.ts
+++ b/src/interpreter/index.ts
@@ -231,10 +231,12 @@ export class Interpreter {
 			return result ?? NULL;
 		} else {
 			const _args = new Map<string, Variable>();
-			for (let i = 0; i < (fn.args ?? []).length; i++) {
-				_args.set(fn.args![i]!, {
+			for (const i of fn.args.keys()) {
+				const argdef = fn.args[i]!;
+				if (!argdef.default) expectAny(args[i]);
+				_args.set(argdef.name, {
 					isMutable: true,
-					value: args[i] ?? NULL,
+					value: args[i] ?? argdef.default!,
 				});
 			}
 			const fnScope = fn.scope!.createChildScope(_args);
@@ -487,7 +489,17 @@ export class Interpreter {
 			}
 
 			case 'fn': {
-				return FN(node.args.map(arg => arg.name), node.children, scope);
+				return FN(
+					await Promise.all(node.args.map(async (arg) => {
+						return {
+							name: arg.name,
+							default: arg.default ? await this._eval(arg.default, scope) : arg.optional ? NULL : undefined,
+							// type: (TODO)
+						};
+					})),
+					node.children,
+					scope,
+				);
 			}
 
 			case 'block': {
diff --git a/src/interpreter/util.ts b/src/interpreter/util.ts
index f3397dfc..f2ba2ce6 100644
--- a/src/interpreter/util.ts
+++ b/src/interpreter/util.ts
@@ -189,7 +189,12 @@ export function reprValue(value: Value, literalLike = false, processedObjects =
 	if (value.type === 'bool') return value.value.toString();
 	if (value.type === 'null') return 'null';
 	if (value.type === 'fn') {
-		return `@( ${(value.args ?? []).join(', ')} ) { ... }`;
+		if (value.native) {
+			// そのうちネイティブ関数の引数も表示できるようにしたいが、ホスト向けの破壊的変更を伴うと思われる
+			return '@( ?? ) { native code }';
+		} else {
+			return `@( ${(value.args.map(v => v.name)).join(', ')} ) { ... }`;
+		}
 	}
 
 	return '?';
diff --git a/src/interpreter/value.ts b/src/interpreter/value.ts
index 90c531b0..3250afd7 100644
--- a/src/interpreter/value.ts
+++ b/src/interpreter/value.ts
@@ -1,4 +1,5 @@
 import type { Node } from '../node.js';
+import type { Type } from '../type.js';
 import type { Scope } from './scope.js';
 
 export type VNull = {
@@ -33,13 +34,18 @@ export type VObj = {
 export type VFn = VUserFn | VNativeFn;
 type VFnBase = {
 	type: 'fn';
-	args?: string[];
 };
 export type VUserFn = VFnBase & {
 	native?: undefined; // if (vfn.native) で型アサーション出来るように
+	args: VFnArg[];
 	statements: Node[];
 	scope: Scope;
 };
+export type VFnArg = {
+	name: string;
+	type?: Type;
+	default?: Value;
+}
 /**
  * When your AiScript NATIVE function passes VFn.call to other caller(s) whose error thrown outside the scope, use VFn.topCall instead to keep it under AiScript error control system.
  */
diff --git a/src/node.ts b/src/node.ts
index 03d737d2..43b556fd 100644
--- a/src/node.ts
+++ b/src/node.ts
@@ -177,6 +177,8 @@ export type Fn = NodeBase & {
 	type: 'fn'; // 関数
 	args: {
 		name: string; // 引数名
+		optional: boolean;
+		default?: Expression; // 引数の初期値
 		argType?: TypeSource; // 引数の型
 	}[];
 	retType?: TypeSource; // 戻り値の型
diff --git a/src/parser/scanner.ts b/src/parser/scanner.ts
index 1bba9fa7..fb2d4c17 100644
--- a/src/parser/scanner.ts
+++ b/src/parser/scanner.ts
@@ -268,6 +268,11 @@ export class Scanner implements ITokenStream {
 					}
 					break;
 				}
+				case '?': {
+					this.stream.next();
+					token = TOKEN(TokenKind.Question, loc, { hasLeftSpacing });
+					break;
+				}
 				case '@': {
 					this.stream.next();
 					token = TOKEN(TokenKind.At, loc, { hasLeftSpacing });
diff --git a/src/parser/syntaxes/common.ts b/src/parser/syntaxes/common.ts
index 9f32c970..f185a158 100644
--- a/src/parser/syntaxes/common.ts
+++ b/src/parser/syntaxes/common.ts
@@ -2,6 +2,7 @@ import { TokenKind } from '../token.js';
 import { AiScriptSyntaxError } from '../../error.js';
 import { NODE } from '../utils.js';
 import { parseStatement } from './statements.js';
+import { parseExpr } from './expressions.js';
 
 import type { ITokenStream } from '../streams/token-stream.js';
 import type * as Ast from '../../node.js';
@@ -12,7 +13,7 @@ import type * as Ast from '../../node.js';
  * ```
 */
 export function parseParams(s: ITokenStream): { name: string, argType?: Ast.Node }[] {
-	const items: { name: string, argType?: Ast.Node }[] = [];
+	const items: { name: string, optional?: boolean, default?: Ast.Node, argType?: Ast.Node }[] = [];
 
 	s.nextWith(TokenKind.OpenParen);
 
@@ -25,13 +26,22 @@ export function parseParams(s: ITokenStream): { name: string, argType?: Ast.Node
 		const name = s.token.value!;
 		s.next();
 
+		let optional = false;
+		let defaultExpr;
+		if ((s.getKind() as TokenKind) === TokenKind.Question) {
+			s.next();
+			optional = true;
+		} else if ((s.getKind() as TokenKind) === TokenKind.Eq) {
+			s.next();
+			defaultExpr = parseExpr(s, false);
+		}
 		let type;
 		if (s.getKind() === TokenKind.Colon) {
 			s.next();
 			type = parseType(s);
 		}
 
-		items.push({ name, argType: type });
+		items.push({ name, optional, default: defaultExpr, argType: type });
 
 		// separator
 		switch (s.getKind()) {
diff --git a/src/parser/token.ts b/src/parser/token.ts
index c07030c7..99af4099 100644
--- a/src/parser/token.ts
+++ b/src/parser/token.ts
@@ -89,6 +89,8 @@ export enum TokenKind {
 	Gt,
 	/** ">=" */
 	GtEq,
+	/** "?" */
+	Question,
 	/** "@" */
 	At,
 	/** "[" */
diff --git a/src/parser/visit.ts b/src/parser/visit.ts
index 29b5cbdf..17d11f0f 100644
--- a/src/parser/visit.ts
+++ b/src/parser/visit.ts
@@ -61,6 +61,11 @@ export function visitNode(node: Ast.Node, fn: (node: Ast.Node) => Ast.Node): Ast
 			break;
 		}
 		case 'fn': {
+			for (const i of result.args.keys()) {
+				if (result.args[i]!.default) {
+					result.args[i]!.default = visitNode(result.args[i]!.default!, fn) as Ast.Fn['args'][number]['default'];
+				}
+			}
 			for (let i = 0; i < result.children.length; i++) {
 				result.children[i] = visitNode(result.children[i]!, fn) as Ast.Fn['children'][number];
 			}
diff --git a/test/index.ts b/test/index.ts
index 22baee6d..9cb52495 100644
--- a/test/index.ts
+++ b/test/index.ts
@@ -591,10 +591,43 @@ describe('Function call', () => {
 		eq(res, NUM(2));
 	});
 
-	test.concurrent('std: throw AiScript error when required arg missing', async () => {
+	test.concurrent('optional args', async () => {
+		const res = await exe(`
+		@f(x, y?, z?) {
+			[x, y, z]
+		}
+		<: f(true)
+		`);
+		eq(res, ARR([TRUE, NULL, NULL]));
+	});
+
+	test.concurrent('args with default value', async () => {
+		const res = await exe(`
+		@f(x, y=1, z=2) {
+			[x, y, z]
+		}
+		<: f(5, 3)
+		`);
+		eq(res, ARR([NUM(5), NUM(3), NUM(2)]));
+	});
+
+	test.concurrent('args must not be both optional and default-valued', async () => {
+		try {
+			Parser.parse(`
+			@func(a? = 1){}
+			`);
+		} catch (e) {
+			assert.ok(e instanceof AiScriptSyntaxError);
+			return;
+		}
+		assert.fail();
+	});
+
+	test.concurrent('missing arg', async () => {
 		try {
 			await exe(`
-			<: Core:eq(1)
+			@func(a){}
+			func()
 			`);
 		} catch (e) {
 			assert.ok(e instanceof AiScriptRuntimeError);
@@ -603,14 +636,16 @@ describe('Function call', () => {
 		assert.fail();
 	});
 
-	test.concurrent('omitted args', async () => {
-		const res = await exe(`
-		@f(x, y) {
-			[x, y]
+	test.concurrent('std: throw AiScript error when required arg missing', async () => {
+		try {
+			await exe(`
+			<: Core:eq(1)
+			`);
+		} catch (e) {
+			assert.ok(e instanceof AiScriptRuntimeError);
+			return;
 		}
-		<: f(1)
-		`);
-		eq(res, ARR([NUM(1), NULL]));
+		assert.fail();
 	});
 });
 
diff --git a/unreleased/optional-args.md b/unreleased/optional-args.md
new file mode 100644
index 00000000..99ffc49d
--- /dev/null
+++ b/unreleased/optional-args.md
@@ -0,0 +1,2 @@
+- 省略可能引数と初期値付き引数を追加。引数名に`?`を後置することでその引数は省略可能となります。引数に`=<式>`を後置すると引数に初期値を設定できます。省略可能引数は初期値`null`の引数と同等です。
+  - BREAKING: いずれでもない引数が省略されると即時エラーとなるようになりました。