diff --git a/etc/aiscript.api.md b/etc/aiscript.api.md
index 7ece385f..ac275635 100644
--- a/etc/aiscript.api.md
+++ b/etc/aiscript.api.md
@@ -123,6 +123,9 @@ function assertBoolean(val: Value | null | undefined): asserts val is VBool;
 // @public (undocumented)
 function assertFunction(val: Value | null | undefined): asserts val is VFn;
 
+// @public (undocumented)
+function assertNull(val: Value | null | undefined): asserts val is VNull;
+
 // @public (undocumented)
 function assertNumber(val: Value | null | undefined): asserts val is VNum;
 
@@ -466,6 +469,9 @@ function isExpression(x: Node_2): x is Expression;
 // @public (undocumented)
 function isFunction(val: Value): val is VFn;
 
+// @public (undocumented)
+function isNull(val: Value): val is VNull;
+
 // @public (undocumented)
 function isNumber(val: Value): val is VNum;
 
@@ -757,18 +763,20 @@ type UnionTypeSource = NodeBase & {
 declare namespace utils {
     export {
         expectAny,
-        assertBoolean,
-        assertFunction,
-        assertString,
-        assertNumber,
-        assertObject,
-        assertArray,
         isBoolean,
         isFunction,
         isString,
         isNumber,
         isObject,
         isArray,
+        isNull,
+        assertBoolean,
+        assertFunction,
+        assertString,
+        assertNumber,
+        assertObject,
+        assertArray,
+        assertNull,
         eq,
         valToString,
         valToJs,
diff --git a/src/interpreter/util.ts b/src/interpreter/util.ts
index ccc5a9ae..33d7488d 100644
--- a/src/interpreter/util.ts
+++ b/src/interpreter/util.ts
@@ -1,6 +1,6 @@
 import { AiScriptRuntimeError } from '../error.js';
 import { STR, NUM, ARR, OBJ, NULL, BOOL } from './value.js';
-import type { Value, VStr, VNum, VBool, VFn, VObj, VArr } from './value.js';
+import type { Value, VStr, VNum, VBool, VFn, VObj, VArr, VNull } from './value.js';
 
 export function expectAny(val: Value | null | undefined): asserts val is Value {
 	if (val == null) {
@@ -8,11 +8,39 @@ export function expectAny(val: Value | null | undefined): asserts val is Value {
 	}
 }
 
+export function isBoolean(val: Value): val is VBool {
+	return val.type === 'bool';
+}
+
+export function isFunction(val: Value): val is VFn {
+	return val.type === 'fn';
+}
+
+export function isString(val: Value): val is VStr {
+	return val.type === 'str';
+}
+
+export function isNumber(val: Value): val is VNum {
+	return val.type === 'num';
+}
+
+export function isObject(val: Value): val is VObj {
+	return val.type === 'obj';
+}
+
+export function isArray(val: Value): val is VArr {
+	return val.type === 'arr';
+}
+
+export function isNull(val: Value): val is VNull {
+	return val.type === 'null';
+}
+
 export function assertBoolean(val: Value | null | undefined): asserts val is VBool {
 	if (val == null) {
 		throw new AiScriptRuntimeError('Expect boolean, but got nothing.');
 	}
-	if (val.type !== 'bool') {
+	if (!isBoolean(val)) {
 		throw new AiScriptRuntimeError(`Expect boolean, but got ${val.type}.`);
 	}
 }
@@ -21,7 +49,7 @@ export function assertFunction(val: Value | null | undefined): asserts val is VF
 	if (val == null) {
 		throw new AiScriptRuntimeError('Expect function, but got nothing.');
 	}
-	if (val.type !== 'fn') {
+	if (!isFunction(val)) {
 		throw new AiScriptRuntimeError(`Expect function, but got ${val.type}.`);
 	}
 }
@@ -30,7 +58,7 @@ export function assertString(val: Value | null | undefined): asserts val is VStr
 	if (val == null) {
 		throw new AiScriptRuntimeError('Expect string, but got nothing.');
 	}
-	if (val.type !== 'str') {
+	if (!isString(val)) {
 		throw new AiScriptRuntimeError(`Expect string, but got ${val.type}.`);
 	}
 }
@@ -39,7 +67,7 @@ export function assertNumber(val: Value | null | undefined): asserts val is VNum
 	if (val == null) {
 		throw new AiScriptRuntimeError('Expect number, but got nothing.');
 	}
-	if (val.type !== 'num') {
+	if (!isNumber(val)) {
 		throw new AiScriptRuntimeError(`Expect number, but got ${val.type}.`);
 	}
 }
@@ -48,7 +76,7 @@ export function assertObject(val: Value | null | undefined): asserts val is VObj
 	if (val == null) {
 		throw new AiScriptRuntimeError('Expect object, but got nothing.');
 	}
-	if (val.type !== 'obj') {
+	if (!isObject(val)) {
 		throw new AiScriptRuntimeError(`Expect object, but got ${val.type}.`);
 	}
 }
@@ -57,33 +85,18 @@ export function assertArray(val: Value | null | undefined): asserts val is VArr
 	if (val == null) {
 		throw new AiScriptRuntimeError('Expect array, but got nothing.');
 	}
-	if (val.type !== 'arr') {
+	if (!isArray(val)) {
 		throw new AiScriptRuntimeError(`Expect array, but got ${val.type}.`);
 	}
 }
 
-export function isBoolean(val: Value): val is VBool {
-	return val.type === 'bool';
-}
-
-export function isFunction(val: Value): val is VFn {
-	return val.type === 'fn';
-}
-
-export function isString(val: Value): val is VStr {
-	return val.type === 'str';
-}
-
-export function isNumber(val: Value): val is VNum {
-	return val.type === 'num';
-}
-
-export function isObject(val: Value): val is VObj {
-	return val.type === 'obj';
-}
-
-export function isArray(val: Value): val is VArr {
-	return val.type === 'arr';
+export function assertNull(val: Value | null | undefined): asserts val is VNull {
+	if (val == null) {
+		throw new AiScriptRuntimeError('Expect null, but got nothing.');
+	}
+	if (!isNull(val)) {
+		throw new AiScriptRuntimeError(`Expect null, but got ${val.type}.`);
+	}
 }
 
 export function eq(a: Value, b: Value): boolean {
diff --git a/unreleased/add-null-assertion.md b/unreleased/add-null-assertion.md
new file mode 100644
index 00000000..4c8d922a
--- /dev/null
+++ b/unreleased/add-null-assertion.md
@@ -0,0 +1 @@
+- null値のアサーション関数（`isNull`, `assertNull`）を追加しました。