PURPOSE: To make it possible to efficiently output a power in supplying the power to an electronic circuit used for an electronic switching system for lighting or the like by applying or cutting off a load by a switch, at the same time switching over the resonant circuit of C and L from a series resonant and vice versa by use of the same switch.

CONSTITUTION: When a switch SW is open, the current flows to a point P through a capacitor C and the primary coil of a transformer L to become a series resonant circuit. Thus, the power generated on the secondary side of the transformer L is utilized to cause an electronic circuit to be actuated. When the switch SW is closed, the current flows to diodes D1 and D2 to generate a voltage across a point B and the point P. Then, the capacitor C and transformer L become a parallel resonant circuit to generate a power on the secondary side of the transformer L. With this power, the electronic circuit can be actuated.