1a and g 2a designate control signal input gates parts associated with the n channel gate g n of the cogmos thyristor 1a are the same as those provided in the third embodiment shown in fig4 parts associated with the p channel gate g p of the cogmos thyristor 1a are such that the cathode k n channel mosfet 39 s and p channel mosfet 39 s in the third embodiment are changed to the anode a p channel mosfet 39 s and n channel mosfet 39 s respectively the operation of this fourth embodiment is similar to that of the third embodiment it is supposed herein that the potential of the cathode k of the cogmos thyristor 1a is not fixed relative to the reference potential of the gate drive circuit that is it is supposed that the potential v k of the cathode k of the cogmos thyristor 1a is in a floating condition first when the potential v k of the cathode k of the cogmos thyristor 1a is lower than the reference potential of the gate drive circuit control signals are applied to the gates g 1 and g 2 so as to turn on the mosfet 39 s 3 6 and to turn off the mosfet 39 s 4 5 a voltage difference equal to the sum of the power supply voltage of the power source 10 and the potential v k of the cathode k of the cogmos thyristor 1a appears across the n channel gate g n and the cathode k of the cogmos thyristor 1a thereby turning on the cogmos thyristor 1a on the other hand when the potential v k of the cathode k of the cogmos thyristor 1a is higher than the reference potential of the drive circuit the.