To solve the problem that a primary smoothing aluminum electrolytic capacitor opens a valve to jet an electrolytic solution when commercial AC power supply is in an overvoltage state.

An aluminum electrolytic capacitor with smaller capacitance than that of a primary smoothing capacitor is connected in parallel to the primary smoothing capacitor, and a thermistor is disposed in the vicinity of the aluminum electrolytic capacitor or contacted thereto. The thermistor is incorporated in a time constant circuit for determining a maximum ON time of a main switching element for turning on and off a primary current on a circuit, or serially connected to a feedback signal line from a transformer secondary winding.