To provide a crime prevention device for a key of a simple structure, capable of preventing picking of the key.

An opening and closing upper cover 2 for covering a keyhole 3 of a door 4 and an alarm device 5 for generating an alarm in the open state of the upper cover 2 are fitted to a cover base 1. When the key of the door 4 is locked, the upper cover 2 is closed to cover the keyhole 3 and fixed not to be easily opened by the stopper 10. The cover base 1 is fixed to the surface of the door by an adhesive or the like. In this state, the projecting part of the upper cover 2 pushes a switch 8 of the alarm device 5 to be ready for generating an alarm, and when the upper cover 2 is opened, the switch 8 is operated to generate an alarm. On the other hand, an alarm operating device 21 is operated to release an alarm, whereby the upper cover 2 is opened to unlock the door.