After mount judging unit (110) transits from a state wherein it decides that earphone (200) is not mounted in an ear to a state wherein it decides that earphone (200) is mounted in the ear, if operating unit (120) accepts a preset unlocking operation, then valid/invalid setter (130) makes a setting for validating operations which will subsequently be accepted by operating unit (120). Valid/invalid setter (130) will keep the setting until mount judging unit (110) decides that earphone (200) is not mounted in the ear. If valid/invalid setter (130) makes the validating setting, controller (140) controls mobile device (100) depending on operations accepted by operating unit (120).