PURPOSE: To prevent the malfunction of a thyristor by providing a means blocking a current toward a gate between the gate of a thyristor to which a capacitive load is connected to a cathode side and a thyristor turn-off switch.

CONSTITUTION: A load driving power supply (not shown) is connected to the anode of a thyristor 1 via a terminal 9 and the capacitive load (not shown) is connected to a cathode via an output terminal 10. In inputting a high level signal to a control input terminal 8, a transistor (TR) 4 is turned on and a TR 2 is turned off to turn on the thyristor 1 and a current is conducted to a load. With a terminal 8 at a low level next, the TR 2 is turned on and the thyristor 1 is turned off. In such case, even when the terminal 10 is at a negative potential, the base current of the TR 2 is not conducted to the gate of the thyristor 1 to prevent the malfunction of the thyristor 1.