PURPOSE: To surely protect it quickly from overcurrents without complicating the circuit constitution by detecting the overcurrent on the source side of a MOSFET, and inverting it and adding it to the gate of the MOSFET.

CONSTITUTION: An overcurrent protective circuit 20 is composed of a detecting resistor 22, which detects a primary winding current on the source side of a power MOSFET 21, and an OP amplifier 25, which receives the detection voltage through the resistor 23 and inverts the detection voltage over the voltage of an offset power source 24 and outputs it to the output side of an amplifier 16 as an output signal. When an overcurrent occurs in external load, the overcurrent protective circuit 20 outputs an inversion signal 20S, and suppresses the driving signal 26S added to the gate G of the power MOSFET 21 to the difference of absolute value between the signals 6S and 20S. Hereby, the surge absorbing element becomes needless.