diff --git a/CHANGELOG.md b/CHANGELOG.md
index ee5b7f78..85af36fa 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@
 - `arr.sort`の処理を非同期的にして高速化
 - `arr.flat`,`arr.flat_map`を追加
 - `Uri:encode_full`, `Uri:encode_component`, `Uri:decode_full`, `Uri:decode_component`を追加
+- `str.starts_with`,`str.ends_with`を追加
 
 # 0.18.0
 - `Core:abort`でプログラムを緊急停止できるように
diff --git a/docs/primitive-props.md b/docs/primitive-props.md
index 6f66d498..37fb6b05 100644
--- a/docs/primitive-props.md
+++ b/docs/primitive-props.md
@@ -62,6 +62,20 @@ Core:range(0,2).push(4) //[0,1,2,4]
 ### @(_v_: str).incl(_keyword_: str): bool
 文字列中に _keyword_ が含まれていれば`true`、なければ`false`を返します。  
 
+### @(_v_: str).starts_with(_prefix_: str, _start\_index_?: num): bool
+文字列が _prefix_ で始まっていれば`true`、そうでなければ`false`を返します。\
+_prefix_ が空文字列の場合は常に`true`を返します。\
+_start\_index_ が指定されている場合、そのインデックスから始めます。\
+_start\_index_ が`v.len`より大きいか`-v.len`より小さい場合は`false`を返します。\
+_start\_index_ が負の場合は末尾から数えます。
+
+### @(_v_: str).ends_with(_suffix_: str, _end\_index_?: num): bool
+文字列が _suffix_ で終わっていれば`true`、そうでなければ`false`を返します。\
+_suffix_ が空文字列の場合は常に`true`を返します。\
+_end\_index_ が指定されている場合、そのインデックスの直前を末尾とします。（省略時は`v.len`）\
+_end\_index_ が`v.len`より大きいか`-v.len`より小さい場合は`false`を返します。\
+_end\_index_ が負の場合は末尾から数えます。
+
 ### @(_v_: str).slice(_begin_: num, _end_: num): str
 文字列の _begin_ 番目から _end_ 番目の直前までの部分を取得します。  
 
diff --git a/src/interpreter/primitive-props.ts b/src/interpreter/primitive-props.ts
index bfc89e55..2ec7476a 100644
--- a/src/interpreter/primitive-props.ts
+++ b/src/interpreter/primitive-props.ts
@@ -120,6 +120,37 @@ const PRIMITIVE_PROPS: {
 			return Number.isNaN(res) ? NULL : NUM(res);
 		}),
 
+		starts_with: (target: VStr): VFn => FN_NATIVE(async ([prefix, start_index], _opts) => {
+			assertString(prefix);
+			if (!prefix.value) {
+				return TRUE;
+			}
+
+			if (start_index) assertNumber(start_index);
+			const raw_index = start_index?.value ?? 0;
+			if (raw_index < -target.value.length || raw_index > target.value.length) {
+				return FALSE;
+			}
+			const index = (raw_index >= 0) ? raw_index : target.value.length + raw_index;
+			return target.value.startsWith(prefix.value, index) ? TRUE : FALSE;
+		}),
+
+		ends_with: (target: VStr): VFn => FN_NATIVE(async ([suffix, end_index], _opts) => {
+			assertString(suffix);
+			if (!suffix.value) {
+				return TRUE;
+			}
+
+			if (end_index) assertNumber(end_index);
+			const raw_index = end_index?.value ?? target.value.length;
+			if (raw_index < -target.value.length || raw_index > target.value.length) {
+				return FALSE;
+			}
+			const index = (raw_index >= 0) ? raw_index : target.value.length + raw_index;
+
+			return target.value.endsWith(suffix.value, index) ? TRUE : FALSE;
+		}),
+
 		pad_start: (target: VStr): VFn => FN_NATIVE(([width, pad], _) => {
 			assertNumber(width);
 			const s = (pad) ? (assertString(pad), pad.value) : ' ';
diff --git a/test/index.ts b/test/index.ts
index 026603fe..cf5fb90e 100644
--- a/test/index.ts
+++ b/test/index.ts
@@ -2628,7 +2628,91 @@ describe('primitive props', () => {
 			);
 		});
 
-		test.concurrent("pad_start", async () => {
+		test.concurrent('starts_with (no index)', async () => {
+			const res = await exe(`
+			let str = "hello"
+			let empty = ""
+			<: [
+				str.starts_with(""), str.starts_with("hello"),
+				str.starts_with("he"), str.starts_with("ell"),
+				empty.starts_with(""), empty.starts_with("he"),
+			]
+			`);
+			eq(res, ARR([
+				TRUE, TRUE,
+				TRUE, FALSE,
+				TRUE, FALSE, 
+			]));
+		});
+
+		test.concurrent('starts_with (with index)', async () => {
+			const res = await exe(`
+			let str = "hello"
+			let empty = ""
+			<: [
+				str.starts_with("", 4), str.starts_with("he", 0),
+				str.starts_with("ll", 2), str.starts_with("lo", 3),
+				str.starts_with("lo", -2), str.starts_with("hel", -5),
+				str.starts_with("he", 2), str.starts_with("loa", 3),
+				str.starts_with("lo", -6), str.starts_with("", -7),
+				str.starts_with("lo", 6), str.starts_with("", 7),
+				empty.starts_with("", 2), empty.starts_with("ll", 2),
+			]
+			`);
+			eq(res, ARR([
+				TRUE, TRUE,
+				TRUE, TRUE,
+				TRUE, TRUE,
+				FALSE, FALSE,
+				FALSE, TRUE,
+				FALSE, TRUE,
+				TRUE, FALSE,
+			]));
+		});
+
+		test.concurrent('ends_with (no index)', async () => {
+			const res = await exe(`
+			let str = "hello"
+			let empty = ""
+			<: [
+				str.ends_with(""), str.ends_with("hello"),
+				str.ends_with("lo"), str.ends_with("ell"),
+				empty.ends_with(""), empty.ends_with("he"),
+			]
+			`);
+			eq(res, ARR([
+				TRUE, TRUE,
+				TRUE, FALSE,
+				TRUE, FALSE,
+			]));
+		});
+
+		test.concurrent('ends_with (with index)', async () => {
+			const res = await exe(`
+			let str = "hello"
+			let empty = ""
+			<: [
+				str.ends_with("", 3), str.ends_with("lo", 5),
+				str.ends_with("ll", 4), str.ends_with("he", 2),
+				str.ends_with("ll", -1), str.ends_with("he", -3),
+				str.ends_with("he", 5), str.ends_with("lo", 3),
+				str.ends_with("lo", -6), str.ends_with("", -7),
+				str.ends_with("lo", 6), str.ends_with("", 7),
+				empty.ends_with("", 2), empty.ends_with("ll", 2),
+			]
+			`);
+			eq(res, ARR([
+				TRUE, TRUE,
+				TRUE, TRUE,
+				TRUE, TRUE,
+				FALSE, FALSE,
+				FALSE, TRUE,
+				FALSE, TRUE,
+				TRUE, FALSE,
+			]));
+		});
+
+    test.concurrent("pad_start", async () => {
 			const res = await exe(`
 			let str = "abc"
 			<: [