To reduce a power consumption by adding a switch between a main power source switch and a stand-by power source, and intermittently supplying a power to the stand-by power source by continuously opening and closing.

A main power source switch 1 is operated, and a subswitch 2 is operated. When the sub-switch 2 is opened, a voltage starts lowering, but a stand-by power source circuit 13 continues operating until an applied voltage is lowered to a stopping voltage. When the voltage is lowered slightly from a reference voltage, an output voltage of a comparator is simultaneously inverted to trigger a one-shot multivibrator to thereby set an output of the multivibrator to an 'H'. Then, the 'H' output of the multivibrator is applied to a relay switch to turn ON the relay switch, and its voltage is raised. When the output of the multivibrator is changed from the 'H' to an 'L', the relay switch is opened, and the voltage starts lowering. When the voltage is lowered from a reference voltage, an output of the comparator is inverted, and the relay switch is repeated ON and OFF. Thus, its switching loss is reduced, thereby decreasing its power consumption.