PURPOSE: To prevent breakdown of a gate control circuit, by using a diode to prevent an AC power source from applying the voltage to the gate control circuit in case of breakdown due to an overcurrent or the like of a TRIAC used as an AC control element.

CONSTITUTION: On a basis of the output stage of a gate control circuit 2, supplying of an ignition pulse to the gate of a TRIAC 5 is controlled, and supplying of the voltage of an AC power source 7 to a load 6 is controlled by the TRIAC 5. In case of an accident where an T1 of the TRIAC is open and the circuit between the gate and an T2 is shorted, a diode 12 prevents the power source voltage from being applied to the output of the gate control circuit 2 of the TRIAC from the AC power source through the load 6 and a current limiting resistance 4 in the half cycle when the load side of the AC power source 7 is positive, and a diode 11 prevents the power source voltage from being applied to the output of gate control circuit of the TRIAC through a DC power source 1 in the half cycle when the load side of the AC power source 7 is negative.