A lock mechanism for locking a playback button plate to its playback position includes a seesaw lever disposed in a plane across the sliding direction of the playback button plate. The seesaw lever has a lock strip capable of engaging the playback button plate which is coupled with a playback button. When the playback button is pressed to retreat the playback button plate into the playback position, the seesaw lever rocks to have its lock strip engage the playback button plate, thereby locking the playback button plate.