A rectifier circuit (201) for converting an A.C.voltage into D.C. is comprised. An output of the rectifier circuit (201) is converted into a high-frequency A.C.voltage by a high-frequency switching circuit (202). An output of the high-frequency switching circuit (202) is converted into a high-frequency sinusoidal wave by a resonance circuit (203). A synchronous detector circuit (204) detects a point of time at which the high-frequency sinusoidal current delivered from the resonance circuit (203) becomes zero. A pulse width control circuit (205) controls the pulse width of the output of the high-frequency switching circuit (202) so that the high-frequency switching circuit (202) may turn «on» at the zero point of the high-frequency sinusoidal current as detected by the synchronous detector circuit (204), and that it may turn «off» when the output current of the resonance circuit (203) has reached a predetermined value.