PURPOSE: To suppress overcurrent, by integrating voltage induced to the tertiary winding of a main transformer, to find magnetic flux density, and by limiting the ON-period of a switching element so that the magnetic flux density may come to magnetic saturation or less.

CONSTITUTION: A fly-back converter is composed of a main transformer 11, a transistor 12 as a switching element 2, a rectifying and smoothing circuit 13, and a control circuit 14. The control circuit 14 is composed of an integrating circuit 18, a comparator 19, an error amplifier 26. a saw-tooth-wave generator 28, a comparator 29, an AND circuit 30, and the like, and when the output signal of the integrating circuit 18 is greater than reference voltage 20 via the comparator 19, then the AND circuit 30 is closed. When DC output voltage is risen, then the pulse width of the output signal of the comparator 29 gets smaller, and the induced voltage of a secondary winding gets lower. Besides, an ON-period lengthened in a state that overcurrent flows is shortened to suppress storage energy.