To return a resonant waveform to the original stable state instantaneously when the abrupt load change has occurred by an L load on the secondary side by detecting the relationship between the off-width and the threshold time of a PWM signal, which controls the switching on the primary side in the abrupt change of the L load.

The output of a synchronization detecting circuit 203 is inputted into a PWM control circuit 201 as a timing synchronization signal. When that the off-operating time of a switching element exceeds a threshold time is detected by the synchronization detecting circuit 203, the second increasing/ decreasing width is selected, and the pulse width is increased. When the off- operating time continuously exceeds the threshold time thereafter, the minimum unit width is selected, and the pulse width is incrased or decreased. When that the off-operating time does not exceed the threshold time again is detected, the second increasing/decreasing width is selected, and the control is performed with the PWM control circuit 201 so as no decrease the pulse width.