PURPOSE: To take ON time of a switching element sufficiently, and to inhibit heat generation by changing over the inductance of a primary winding for an oscillation transformer in a charging circuit according to the magnitude of load.

CONSTITUTION: In a charging circuit with an inverter 1 including an oscillation transformer and a switching element 9, the inductance of a primary winding for the oscillation transformer is changed over. For changing over said induct ance, primary windings 8 and 24 having different inductance may be mounted and changed over by a switch or an intermediate tap set up to the primary winding 8 and short-circuited by the switch. The switch 25 is changed over when load 17 is supplied with electric power at the same time as a battery 2 is charged only. Accordingly, ON time of the switching element 9 can be set in proper length at all times, thus preventing heat generation.