PURPOSE: To simplify a driving circuit and improve switching characteristics, by cascade-connecting photosensitive devices such as electrostatic induction phototransistor to an electrostatic induction thyristor and connecting a gate to a drain between the photosensitive devices and connecting a zener diode.

CONSTITUTION: A primary electrostatic induction phototransistor (SIPT) Q13 is Darlington-connected to an auxiliary SIPT Q14 and is cascade-connected to an electrostatic induction thyristor (SI) Q10. A cathode of the SI Q10 is connected to a source commonly connected to the SIPTs Q13 and Q14 of N channel, and a drain of the SIPT Q14 is connected to a gate of the SIPT Q13, and an anode and a cathode of a zener diode ZD is respectively connected between a gate of the SI Q10 and the source of the SIPT Q13. Resistors R2 and R3 are respectively connected between the gate and drain of the SIPTs Q13 and Q14. Accordingly, a variable control current can be increased.