PURPOSE: To provide the burglar prevention device for the time of engine suspension and of idling operations at low cost, and let the device be functioned by simple operations.

CONSTITUTION: The burglar prevention device against the insertion of an informal key is made up of a formal key insertion detector 1, an engine revolution detecting circuit 2, a burglar prevention circuit 3, an alarm 4, an engine control unit 5 and of a relay 7, and a burglar prevention device for the time of engine idling operations is made up of the formal key insertion detector 1, the engine revolution detecting circuit 2, the burglar prevention circuit 3, the alarm 4, the engine control unit 5, the relay 7, a door lock detector 8, and of a side brake detector 9 as shown by the figure 4. By this constitution, the inexpensive, simple and effective burglar prevention device capable of positively preventing an automobile burglar, can thereby be provided.