PURPOSE: To stabilize operation of an apparatus by applying a reverse bias from an output filter circuit to a main switching circuit, when said main switching circuit is changed to its OFF state.

CONSTITUTION: A switching type power supply is composed of an input filter circuit 1 consisting of a capacitor 11, a main switching circuit 2 having a transistor(Tr) 21 turning ON and OFF an input voltage, a drive circuit 5 for said Tr 21, a control circuit 6 generating a control signal, and an auxiliary power circuit 4. In this case, an output filter circuit 3 is composed of C33 between output terminals, a choke coil 32 generating a reverse bias electromotive force by an output DC current from the TR 21, and D34. Thus, when a signal from the drive circuit 5 is turned OFF, the TR 21 is changed to its OFF state and, at that time, an electromotive force applied in the direction of the arrow shown in the drawing is induced in the choke coil 32 to apply a reverse bias between the base and emitter of the TR 21.