PURPOSE: To improve the efficiency as the entire gate circuit and the reliability by providing separately a transistor (TR) circuit for higher gate conducting a GTO thyristor and a TR circuit keeping the conductive state and connecting them in parallel.

CONSTITUTION: In order to make the GTO thyristor 1 conductive, an ON/OFF signal generating circuit 4 responds to a control signal from a control system 5, generates an ON signal, gives to a TRQ22 of a turn-on circuit and to a higher gate pulse generating circuit 6. The pulse generating circuit 6 generates a pulse signal having a pulse width corresponding to the higher gate period as soon as the ON signal enters and gives the signal to a TRQ21, the TRQ21 is conductive, a current from the power supply 21 flows through a reistor 21 and the TRQ21 and to a gate of the GTO thyristor 1. In selecting the resistance value of the resistor R21 limiting the current to a proper value, much current having a steep leading flows to the gate.