To enable stable power supply by switching a discharge resistance parallel-connected with a load and thereby controlling a voltage supplied to the load to be a certain value or below, even if the load on a secondary circuit fluctuates.

A power transistor Q3 is brought into conduction by a transistor Q2, and a load as seen from the secondary coil of power supply is made to increase through a discharge resistance R9. A smoothing circuit for re-smoothing a voltage pulsating in a discharge circuit is constituted of an inductance L1 and a capacitor C4, and a smoothed output is supplied to the load. The output voltage of a resonance circuit is highest when there is no load. When there is no load or there is very small slight load, the voltage is controlled through the discharge resistance R9 to have the ratio of on to off of the transistor Q3 varied, and the average output voltage is thereby kept constant. As a result, even if resonance is used for the secondary circuit and the voltage accompanying the load fluctuate due to the fluctuations in the load, the voltage supplied to the load is controlled to a constant value or lower.