diff --git a/lib/devbase/volume/compose.py b/lib/devbase/volume/compose.py
index b907036..0c00fc0 100644
--- a/lib/devbase/volume/compose.py
+++ b/lib/devbase/volume/compose.py
@@ -203,6 +203,9 @@ def generate_scaled_compose(
         if service_name != dev_service_name:
             copied = _deep_copy(service_config)
             _rewrite_depends_on(copied, dev_service_name, scale)
+            # Insert tini as PID 1 so orphaned children are reaped (no zombies).
+            # setdefault keeps an explicit `init: false` if the project set one.
+            copied.setdefault('init', True)
             scaled_config['services'][service_name] = copied
 
     # Generate a service for each instance
@@ -216,6 +219,10 @@ def generate_scaled_compose(
         # Update container name
         service['container_name'] = f"${{COMPOSE_PROJECT_NAME}}-{dev_service_name}-{i}"
 
+        # Insert tini as PID 1 so orphaned children are reaped (no zombies).
+        # setdefault keeps an explicit `init: false` if the project set one.
+        service.setdefault('init', True)
+
         # Remove environment section (use env_file instead to avoid exposing secrets)
         if 'environment' in service:
             del service['environment']
diff --git a/tests/volume/__init__.py b/tests/volume/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/tests/volume/test_compose.py b/tests/volume/test_compose.py
new file mode 100644
index 0000000..da7f22c
--- /dev/null
+++ b/tests/volume/test_compose.py
@@ -0,0 +1,74 @@
+"""compose.py: generate_scaled_compose() の `init: true` 注入挙動 (issue #28)
+
+devbase コンテナの PID 1 は entrypoint の `tail -f /dev/null` であり orphan を reap しない。
+docker の `init: true` で tini を PID 1 に挿入しゾンビ蓄積を防ぐため、生成される全サービスに
+`init` を注入する。ユーザーが明示した `init: false` は setdefault で尊重する。
+"""
+
+from __future__ import annotations
+
+import os
+
+import yaml
+import pytest
+
+from devbase.volume import compose
+
+
+@pytest.fixture
+def in_tmp_cwd(tmp_path, monkeypatch):
+    """生成物 (.docker-compose.scale.yml) が散らからないよう CWD を tmp に移す。"""
+    monkeypatch.chdir(tmp_path)
+    # DEV_SERVICE_NAME が外部環境に左右されないよう既定に固定
+    monkeypatch.delenv("DEV_SERVICE_NAME", raising=False)
+    return tmp_path
+
+
+def _write_compose(tmp_path, services: dict) -> None:
+    (tmp_path / "compose.yml").write_text(
+        yaml.safe_dump({"services": services}, sort_keys=False),
+        encoding="utf-8",
+    )
+
+
+def _load_scaled(tmp_path) -> dict:
+    return yaml.safe_load((tmp_path / ".docker-compose.scale.yml").read_text())
+
+
+def test_dev_and_non_dev_services_get_init_true(in_tmp_cwd):
+    """dev インスタンスと non-dev サービスの双方に init: true が注入される。"""
+    _write_compose(in_tmp_cwd, {
+        "dev": {"image": "dev:latest"},
+        "mysql": {"image": "mysql:8"},
+    })
+
+    compose.generate_scaled_compose(scale=1, project_name="proj")
+    scaled = _load_scaled(in_tmp_cwd)["services"]
+
+    assert scaled["dev-1"]["init"] is True
+    assert scaled["mysql"]["init"] is True
+
+
+def test_init_injected_for_every_scaled_instance(in_tmp_cwd):
+    """scale>1 でも各 dev-i 全てに init: true が付く。"""
+    _write_compose(in_tmp_cwd, {"dev": {"image": "dev:latest"}})
+
+    compose.generate_scaled_compose(scale=3, project_name="proj")
+    scaled = _load_scaled(in_tmp_cwd)["services"]
+
+    for i in (1, 2, 3):
+        assert scaled[f"dev-{i}"]["init"] is True
+
+
+def test_explicit_init_false_is_preserved(in_tmp_cwd):
+    """明示的な init: false は setdefault により上書きされない。"""
+    _write_compose(in_tmp_cwd, {
+        "dev": {"image": "dev:latest", "init": False},
+        "mysql": {"image": "mysql:8", "init": False},
+    })
+
+    compose.generate_scaled_compose(scale=1, project_name="proj")
+    scaled = _load_scaled(in_tmp_cwd)["services"]
+
+    assert scaled["dev-1"]["init"] is False
+    assert scaled["mysql"]["init"] is False