PURPOSE: To eliminate the necessity of increasing power capacity of a resistor, to reduce a component mounting area and to reduce a cost by connecting a collector of a switching transistor to a base of a control transistor, an emitter to the other side of a power source and a base to an emitter of the control transistor through a resistor.

CONSTITUTION: If a normal current flows to a main transistor 1, values of a resistor 4 and a base resistor 8 of a switching transistor 7 are set so as to turn OFF the transistor 7. Accordingly, when an overcurrent flows to an emitter of the transistor 1, a base current of the transistor 1 is increased. Thus, the transistor 7 is turned ON, a control transistor 3 is turned OFF, the base current of the transistor 1 is cut OFF, and an overcurrent from the transistor 1 to a load 9 is cut OFF. Thus, a component mounting area can be reduced without increasing power capacity of the resistor 4, and its cost can be reduced.