PURPOSE: To eliminate an output voltage variation by detecting a quantity of load current, deducting said quantity from a quantity of maximum load current and causing a differential current to flow in parallel with both ends of a Zener diode.

CONSTITUTION: In a power circuit, a transistor (hereinafter referred to as Tr) 3 is turned ON by an oscillating circuit 1 so that energy is stored in a choke coil 2, and electric current is caused to flow through a Zener diode 7 and a load 6 via a capacitor 4 and a diode 9 when the Tr 3 is turned OFF. In this case, a circuit 5 for detecting a quantity of load current and causing an electric current as much as deduction of said detected quantity from a quantity of maximum load current to flow is juxtaposed with the load 6. Thus, if electric current flows through said circuit 5, the sum of said current and an electric current flowing through the load 6 is always the maximum load current. As a result, electric current flowing through the Zener diode 7 is also held constant.