PURPOSE: To minimize capacity of power supply by providing a current transformer detecting AC current of a main circuit and a rectifier converting an output of the current transformer into a DC, and supplying the output of the transformer to a negative gate current power supply of a self turn off type thyristor.

CONSTITUTION: When an electrostatic induction thyristor (SITY) 1 starts an opening and closing operation, a negative gate current is supplied to the SITY 1 from a power supply 3 in every extinction of arc, so that the current having a some value is outputted. While, as an electric current of a main circuit is supplied to an AC line CC, the current is also supplied to a secondary line of a current transformer 4, and it is converted into DC by a rectifier 5 to input to the power supply 3. The input current is decided by a current ratio of the current of the AC line CC and the current transformer 4, and a stable operation is possible because of depending on no voltage of the power supply 3. Accordingly, the power supply 3 may have a current capacity of the difference between the current to be outputted to the SITY 1 and the current to be inputted from the current transformer 4. According to this constitution, the capacity of the power supply 3 can be minimized by setting a current ration of the current transformer 4 properly.