A controlling unit generates a pulse signal whose duty ratio is controlled so that the output voltage of each channel will be constant, and controls the main transistor and the rectification circuit of each channel in accordance with the pulse signal. The first synchronous rectification transistor is disposed in a direction such that the cathode of the body diode thereof is on the switching terminal side. The second synchronous rectification transistor is disposed in a direction such that the cathode of the body diode thereof is on the output terminal side. The controlling unit first turns on the first synchronous rectification transistor, and then turns on the second synchronous rectification transistor for a period of time in accordance with the pulse width of the pulse signal.