PURPOSE: To suppress the distortion of output voltage waveform of a high frequency transformer by quickening the timing of the switching operation of two switching elements, which perform switching with their phases slid from a group of switches, by the period equivalent to the dead time.

CONSTITUTION: In a switching sequence which generates high-frequency power by alternately turning on and off switching elements 11 and 12 constituting an inverter part, and alternately turning on and off, switching elements 13 and 14 performing switching, with their phases slid from this switching element group, the switching operation of the switching elements 13 and 14 are quickened by the period equivalent to the dead time Td. For this account, the output voltage er of a high frequency transformer is corrected into waveform where the low-order high frequency is reduced. Hereby, the distortion of the output voltage waveform of the high frequency transformer can be suppressed.