PURPOSE: To simplify the selection of fuses and to protect overcurrent without changing the characteristics of a circuit by detecting the voltage drop by overcurrent of a semiconductor switch for turning on/off the power output line of a power circuit having a control terminal, and stopping the operation of the power circuit.

CONSTITUTION: The saturated on-resistance of an FET 9 is considered to be 0.3Ω for example, and maximum load current is to be 1A for convenience. On this occasion, when voltage is supplied from the output of the FET 9, and the FET 9 and a transistor(Tr) 10 are on, and the output current of the FET 9 becomes 3A for example, a voltage drop of 0.9V is generated in the FET 9. Since a Tr 11 is turned on and gets a saturation voltage of about 0.1V at this time, a voltage of about 0.8V is applied between the base and the emitter of a Tr 12. Accordingly, the Tr 12 can be turned on sufficiently, and VCC1 of a PWM-IC 1 is the output voltage of a switching circuit or more. Consequently, a Tr 23 is turned on by the drive of the base of a Tr 14. As the result of this, the operation of the Ic 1 is stopped, and the FET 9 is turned off.