PURPOSE:To improve the reliability by constituting an overcurrent protecting circuit of a conductive modulation type (FET), of a drain-source voltage detecting circuit of the FET, and a circuit for dropping a gate-source voltage of the FET by an output of said circuit. CONSTITUTION:When a short circuit accident is generated in a load 3, and an overcurrent flows to a conductive modulation type MOSFET 1, and a voltage generated in a resistance 13 of a voltage detecting circuit exceeds a Zener voltage of a Zener diode 16, a thyristor SCR 14 is turned on. In this case, a MOSFET 15 is inserted in advance in series to the thyristor 14, therefore, a delay time of a delaying circuit consisting of a floating capacity of this gate and a resistance 19 is made longer than a turn-on time of the FET 1. As a result, a high gate voltage can be applied to the FET except flowing of an overcurrent to the FET 1, and a turn-on failure is prevented. Also, when the SCR 14 becomes on, a gate-source voltage of the FET 1 becomes the sum of the SCR 14 and the MOSFET 15, and becomes below the minimum gate voltage easily.