PURPOSE: To reduce the number of parts of an apparatus and to contrive to lower its cost by broadening the width of an ON pulse to a switching means, when an input voltage is low, and on the other hand, by narrowing the width of said on pulse to the switching means, when said input voltage is high.

CONSTITUTION: When a diode bridge 11 rectifies an input voltage, the rectified voltage is intermittently supplied to a transformer T by a transistor 12 and, after voltage transformation, is supplied to the secondary side. In this case, an ON-pulse width controller 13 controls the ON-pulse width of said transistor 12 on the basis of an electric current flowing through the transistor 12, a voltage Vc1 rectified by said diode bridge 11 and an output voltage VIN on the secondary side. Therefore, the width of an ON pulse to the transistor 12 is broadened when an input voltage from an AC input power supply 10 is low, and on the other hand, the width of said ON pulse to the transistor 12 is narrowed when said input voltage is high.