PURPOSE: To prevent the malfunctions of a microcomputer by avoiding such a case where the voltage supplied to the microcomputer drops less than a back-up assurance level in a period when the drop of voltage due to the cut-off of a power supply is detected and the microcomputer is switched to a low power consumption mode.

CONSTITUTION: The driving power is supplied to a microcomputer 3 from a power circuit 1 via a reverse current preventing circuit 2. Then a parallel circuit which consists of a current limit resistor 6 and a diode 7 and is connected to the power input part of the microcomputer 3 through one of both ends is added together with a back-up capacitor 8 which is connected to the other end of the parallel circuit. The cathode of the diode 7 is connected to the microcomputer 3, and the drop of the power voltage is detected by a voltage detecting circuit 4 connected to the circuit 1 when the circuit 1 is cut off. Then the microcomputer 3 is changed to a low power consumption mode, and the voltage charged in the capacitor 8 is supplied to the microcomputer 3 for back-up via the parallel circuit consisting of the resistor 6 and the diode 7.