PURPOSE: To protect elements from overvoltages sufficiently and to prevent erroneous ignition even if noise currents enter, by providing a gate lead-out structure, and imparting an overvoltage protecting function, which is controlled by an external electric signal.

CONSTITUTION: An avalanche current is generated at a specified limited place and controlled to a minute avalanche current so as not to cause element breakdown due to overheat. Said avalanche current acts as a gate current for a first-stage pilot thyristor 21a. The pilot thyristor 21a is turned ON, and a second-stage pilot thyristor 21b and a main thyristor 21m are also immediately turned ON. In this structure, a curved-in part 28 is provided at the central part of a substrate, where the junction of an N- base layer and P-base layer is present. The gate sensitivity of the first-stage pilot thyristor is made high just like a pilot thyristor for an optical thyristor. In this way, the avalanche current, which is generated when a forward overvoltage is applied, is utilized as the gate current, and a thyristor with an overvoltage protecting function 21 is safely turned ON, and the overvoltage protecting function is provided.