A rectifying circuit (18) converts inputted AC power to DC power. An inverter circuit (22) converts the DC power to high-frequency power in accordance with a switching control signal applied thereto from a control circuit (50). A voltage transformer (24) voltage-transforms the voltage of the high-frequency power. An output-side rectifying circuit (40) rectifies the voltage-transformed power. An output detecting circuit (34) detects the magnitude of the voltage of the rectified power, and a signal representative of the detected voltage is applied to the control circuit (50). The control circuit (50) generates such a switching control signal as to make the rectified power have a predetermined value. The value of the voltage from the rectifying circuit (18) is detected by an input detecting circuit (62), and a signal representative of the detected voltage is applied to the control circuit (50). The control circuit (50) controls the operation of the inverter circuit (22) in accordance with the detected voltage representative signal from the input detecting circuit (62) and the saturation magnetic characteristic of the voltage-transformer (24).