PURPOSE: To reduce influences applied to a power source itself by generating a short-circuiting state.

CONSTITUTION: When terminals A and B on the side of a load 6 are short- circuited, an overcurrent I flows, this overcurrent I is detected by a resistor R2, and a transistor Tr3 is turned on. Then, since a transistor Tr1 is turned off and a transistor Tr1 is turned off as well, no current flows. When the current stops flowing, the transistor Tr1 is turned on as well, and the current flows to the load 6. When the current starts flowing, these operations are repeated. Then, time to the ON of the transistor Tr2 is set by the time constant of a resistor R1 and a capacitor C1. Corresponding to this set time constant, the duty ratio of turning on/off the transistor Tr1 due to the ON of the transistor Tr2 is set at a certain degree so as to eliminate influences such as heating at the transistor Tr1.