PURPOSE: To suppress the dielectric strength of an SCR low and simplify the constitution of the switch device by connecting a nonlinear resistance element to the SCR in parallel and suppress an overvoltage, and disconnecting a mechanical switch from a circuit when the SCR is off.

CONSTITUTION: The nonlinear resistor 4 is connected to the SCR switch 2 of the switch device in parallel, a mechanical switch 5 for opening the SCR is connected between the parallel circuit and a capacitor 3, and an AC current from an AC voltage source 1 to the capacitor 3 is intermitted by the switch 2. When this device connects the capacitor 3 to the voltage source 1, the switch 5 is turned off with the swtich 2 off to impress a source voltage E to the switch 2, and when the voltage impressed to the switch 2 is at "0" V a time t1 later, the switch 2 is turned on with a control signal to connect the capacitor 3 to the voltage source 1. When the capacitor is disconnected, the gate signal of the switch 2 is turned off and the switch 2 is turned off at the time t3 when the flowing current is ceased. Then, the dielectric strength of the switch 2 is lowered and the constitution of the device is simplified.