PURPOSE: To prevent erroneous operation of a drive circuit by providing a voltage monitoring circuit for monitoring supply voltage of a source circuit and turning a switching means OFF, when the supply voltage is lower than a predetermined level, to disable voltage supply to the drive circuit.

CONSTITUTION: A switching means, i.e., a transistor 41, is arranged between a drive circuit 11 and a power source circuit 31, and a voltage monitoring means 51 for monitoring supply voltage of the power source circuit 31 and turning the transistor 41 OFF, when the supply voltage is lower than a predetermined level, to disable voltage supply to the drive circuit 11 is provided. The transistor 41 has emitter connected with the power source circuit 31 and collector connected with the buzzer 12 in the drive circuit 11. By such arrangement, drive voltage is not fed to the drive circuit 11 if the supply voltage from the power source circuit 31 is lower than a predetermined level when the power source is turned ON, for example.