To easily detect picking and to operate a crime preventing device in a reliable manner.

A door 1 has a keyhole 4 of a cylinder lock, and the inner part of the keyhole 4 is provided with an optical sensor 10 detecting the tip part of a duplicate key 7. A crime preventive buzzer 2 sounding when a picking tool is inserted in a keyhole 15 is connected to the optical sensor 10. The crime preventive buzzer 2 is put out of alarm when a signal from the optical sensor 10 is input. The optical sensor 10 and the crime preventive buzzer 2 are put in a security ON state operable simultaneously with locking.