To prevent breaking of a lock by issuing an alarm when a lock with a key hole is being destroyed.

A lock breakage preventing alarm device comprises a lock 3 with a key hole 31, a switch 5 for carrying current to an alarm 6 installed in front of the lock 3, and an alarm circuit 7 including the switch 5, the alarm 6, and a power supply. The switch 5 has one or plural pairs of contact conducting members 54, 57 provided while sandwiching an insulating member 55, a through hole 500 matching in phase with the key hole 31 in the lock 3 and being as narrow as the key hole 31 is bored in the switch 5, and when a lock destroyer such as a drill bites into the switch the destroyer passes through the insulating member 55 and makes contact with the pair of conducting members 54, 56 sandwiching the second insulating member 55, causing the switch of the alarm circuit 7 to turn on to operate the alarm 6.