Abstract: In a bidirectional photothyristor formed on a single N type silicon substrate, a distance between a P-gate diffusion region of one thyristor and an anode diffusion region of another thyristor opposed thereto is set to be 40 to 1,000 .mu.m, preferably, 70 to 600 .mu.m, thereby eliminating a malfunction caused by a noise due to a differentiation circuit which is composed of parasitic resistors and junction capacitances. In a field portion between the P-gate diffusion region and the anode diffusion region, an oxygen-doped semi-insulating film is formed via an SiO.sub.2 film, and an Al conductor is removed to form a field light receiving portion. Unlike a P-gate light receiving portion formed in the P-gate diffusion region, the field light receiving portion does not involve a junction capacitance. Therefore, a light sensitivity can be enhanced without lowering a dV/dt resistance.