To provide an inexpensive crime prevention device for preventing the lock of a doorknob, etc., from being broken.

This device is provided with an insertion pin 50 which can freely be inserted into the keyhole of the doorknob 54 or is preliminarily installed, and when the pin 50 is extracted from the keyhole, removed or brought into contact with an object while a release code signal is not inputted, an insertion pin change detecting means detects this and a control circuit 22 generates an alarm signal according to an internally written alarm program on the basis of a detection signal of the insertion pin change detecting means. An electric sound converter 30 emits an alarm sound on the basis of the alarm signal. A telephone line connection circuit 72 is also operated to automatically make a call to a superintendent room and a guard room and an alarm message is transmitted.