diff --git a/src/interpreter/lib/math.ts b/src/interpreter/lib/math.ts
index 691753b0..a4c75d2d 100644
--- a/src/interpreter/lib/math.ts
+++ b/src/interpreter/lib/math.ts
@@ -228,7 +228,7 @@ export const stdMath: Record<`Math:${string}`, Value> = {
 			}
 			case 'chacha20': {
 				if (!isSecureContext) throw new AiScriptRuntimeError(`The random algorithm ${algo} cannot be used because \`crypto.subtle\` is not available. Maybe in non-secure context?`);
-				return await GenerateChaCha20Random(seed);
+				return await GenerateChaCha20Random(seed, options?.value);
 			}
 			default:
 				throw new AiScriptRuntimeError('`options.algorithm` must be one of these: `chacha20`, `rc4`, or `rc4_legacy`.');
diff --git a/src/utils/random/genrng.ts b/src/utils/random/genrng.ts
index fff2ee9e..bb988d02 100644
--- a/src/utils/random/genrng.ts
+++ b/src/utils/random/genrng.ts
@@ -3,7 +3,7 @@ import { FN_NATIVE, NULL, NUM } from '../../interpreter/value.js';
 import { textEncoder } from '../../const.js';
 import { SeedRandomWrapper } from './seedrandom.js';
 import { ChaCha20 } from './chacha20.js';
-import type { VNativeFn, VNum, VStr } from '../../interpreter/value.js';
+import type { Value, VNativeFn, VNum, VStr } from '../../interpreter/value.js';
 
 export function GenerateLegacyRandom(seed: VNum | VStr): VNativeFn {
 	const rng = seedrandom(seed.value.toString());
@@ -26,11 +26,17 @@ export function GenerateRC4Random(seed: VNum | VStr): VNativeFn {
 	});
 }
 
-export async function GenerateChaCha20Random(seed: VNum | VStr): Promise<VNativeFn> {
+export async function GenerateChaCha20Random(seed: VNum | VStr, options: Map<string, Value> | undefined): Promise<VNativeFn> {
 	let actualSeed: Uint8Array;
-	if (seed.type === 'num')
-	{
-		actualSeed = new Uint8Array(await crypto.subtle.digest('SHA-384', new Uint8Array(new Float64Array([seed.value]))));
+	if (seed.type === 'num') {
+		const float64Array = new Float64Array([seed.value]);
+		const numberAsIntegerOptionValue = options?.get('chacha20NumberSeedLegacyBehaviour');
+		let numberAsInteger = false;
+		if (numberAsIntegerOptionValue?.type === 'bool') {
+			numberAsInteger = numberAsIntegerOptionValue.value;
+		}
+		const seedToDigest = numberAsInteger ? new Uint8Array(float64Array) : new Uint8Array(float64Array.buffer);
+		actualSeed = new Uint8Array(await crypto.subtle.digest('SHA-384', seedToDigest));
 	} else {
 		actualSeed = new Uint8Array(await crypto.subtle.digest('SHA-384', new Uint8Array(textEncoder.encode(seed.value))));
 	}
diff --git a/test/std.ts b/test/std.ts
index 22598d13..105475dd 100644
--- a/test/std.ts
+++ b/test/std.ts
@@ -2,7 +2,7 @@ import * as assert from 'assert';
 import { describe, expect, test } from 'vitest';
 import { utils } from '../src';
 import { AiScriptRuntimeError } from '../src/error';
-import { NUM, STR, NULL, ARR, OBJ, BOOL, TRUE, FALSE, ERROR ,FN_NATIVE } from '../src/interpreter/value';
+import { NUM, STR, NULL, ARR, OBJ, BOOL, TRUE, FALSE, ERROR, FN_NATIVE } from '../src/interpreter/value';
 import { exe, eq } from './testutils';
 
 
@@ -88,7 +88,7 @@ describe('Math', () => {
 	test.concurrent('max', async () => {
 		eq(await exe("<: Math:max(-2, -3)"), NUM(-2));
 	});
-	
+
 	/* flaky
 	test.concurrent('rnd', async () => {
 		const steps = 512;
@@ -158,6 +158,35 @@ describe('Math', () => {
 		eq(res, ARR([BOOL(true), BOOL(true)]));
 	});
 
+	test.concurrent('gen_rng number seed', async () => {
+		// 2つのシード値から1~maxの乱数をn回生成して一致率を見る(numがシード値として指定された場合)
+		const res = await exe(`
+		@test(seed1, seed2) {
+			let n = 100
+			let max = 100000
+			let threshold = 0.05
+			let random1 = Math:gen_rng(seed1)
+			let random2 = Math:gen_rng(seed2)
+			var same = 0
+			for n {
+				if random1(1, max) == random2(1, max) {
+					same += 1
+				}
+			}
+			let rate = same / n
+			if seed1 == seed2 { rate == 1 }
+			else { rate < threshold }
+		}
+		let seed1 = 3.0
+		let seed2 = 3.0000000000000004
+		<: [
+			test(seed1, seed1)
+			test(seed1, seed2)
+		]
+		`)
+		eq(res, ARR([BOOL(true), BOOL(true)]));
+	});
+
 	test.concurrent('gen_rng should reject when null is provided as a seed', async () => {
 		await expect(() => exe('Math:gen_rng(null)')).rejects.toThrow(AiScriptRuntimeError);
 	});
@@ -202,7 +231,7 @@ describe('Obj', () => {
 
 		<: Obj:merge(o1, o2)
 		`);
-		eq(res, utils.jsToVal({ a: 1, b: 3, c: 4}));
+		eq(res, utils.jsToVal({ a: 1, b: 3, c: 4 }));
 	});
 
 	test.concurrent('pick', async () => {
diff --git a/unreleased/chacha20-seed-unexpected-rounding-fix.md b/unreleased/chacha20-seed-unexpected-rounding-fix.md
new file mode 100644
index 00000000..8387aa18
--- /dev/null
+++ b/unreleased/chacha20-seed-unexpected-rounding-fix.md
@@ -0,0 +1,3 @@
+- Fix: **Breaking Change** `Math:gen_rng`の`seed`に`num`を与え、`options.algorithm`に`chacha20`を指定した或いは何も指定しなかった場合、`seed & 255`を内部的に`seed`としてしまう問題を修正。
+  - 関数`Math:gen_rng`の`options.chacha20NumberSeedLegacyBehaviour`に`true`を指定した場合、修正前の動作をする機能を追加(デフォルト:`false`)。
+  - これらの修正により、同じ`seed`でも修正前と修正後で生成される値が異なるようになります。