PURPOSE: To avoid the waste of a power supply, by supplying the power for the start of a DC-DC converter circuit as well as to a prescribed control system and then discontinuing the power supply to these above-mentioned converter circuit and the control system after the lapse of a desired time.

CONSTITUTION: When a start switch 16 of a power supply circuit 22 is applied, a power supply waste preventing circuit 13 starts the working of a DC-DC converter circuit 2 and at the same time supplies a power supply 1 to an ultra-low frequency oscillating circuit 14 functioning as a timer, a tank circuit 26, etc. Then the power supply 1 is discontinued to the circuit 2 after the lapse of a prescribed period of time that is set by the circuit 14. At the same time, the operation of the circuit 2 is focibly stopped. In such way, the waste of the power supply 1 can be assuredly prevented with extremely simple constitution of circuit and without using any nagnet.