A secondary-side rectifying and smoothing circuit rectifies and smoothes an output voltage from a secondary coil of a transformer and outputs a rectified and smoothed voltage to the outside. A tertiary-side rectifying and smoothing circuit rectifies and smoothes an output voltage from a tertiary coil to produce a direct-current voltage and detects and outputs the direct-current voltage as a detected voltage of the output voltage from the secondary-side rectifying and smoothing circuit. A control circuit controls the switching operation of a main switching device on the basis of the detected voltage so that the output voltage is stabilized. The secondary-side rectifying and smoothing circuit includes a rectification-side synchronous rectifier and a commutation-side synchronous rectifier as rectifying devices. The tertiary-side rectifying and smoothing circuit includes a commutation-side synchronous rectifier as a rectifying device that rectifies the output voltage from the tertiary coil, the commutation-side synchronous rectifier being switched on when the main switching device is turned off.