PURPOSE: To prevent turning-off of system due to transient source voltage drop, in an electronic machinery having a voltage drop detecting circuit, by providing a circuit for nullifying the output from the voltage drop detecting circuit only when the system is turned on.

CONSTITUTION: A voltage drop detecting circuit 10 for monitoring the voltage of a battery 1 and detecting droppage of the battery voltage to the limit voltage for normal operation of an electronic machinery employing the battery 1 as power source is provided. A detection signal is fed through a chip enable 16 to a system on/off control circuit 17 so as to turn a system off normally based on the detection signal fed from the detection circuit 10. When the system is started up the detection signal is nullified by the chip enable 16. Consequently, turning off of the system can be prevented when the battery 1 voltage drops temporarily due to rush current at starting up of the system.