PURPOSE: To reduce the number of thyristors by a method wherein an arrester is combined to series connection of VBO free thyristors.

CONSTITUTION: VBO free thyristors 1A are connected in series or in series- parallel connection, and the series number of the thyristors 1A is set by reverse withstand voltage of the thyristors 1A. An arrester 5 is connected between terminals A, K of a thyristor valve in such constitution. The VBO free thyristor 1A having self-tiring function as a main thyristor is fired itself at overvoltage of definite value or more without light gate signal from outside so as to protect the thyristor 1A.