A firing circuit for two antiparallel-connected power thyristors. Each of the power thyristors has a gate terminal; the gate terminals being connected by a series combination of auxiliary thyristors. Each of the auxiliary thyristors is of the type which can withstand firing resulting from excessive anode cathode potential; the sum total of the anode cathode potentials of the series-connected auxiliary thyristors being less than the anode-cathode breakdown voltages of the power thyristors. This, while a power thyristor is in a nonconductive state, an anode-cathode voltage is impressed thereacross in the forward conduction direction. At an anode-cathode voltage which is less than the breakdown voltage of the power thyristor, the auxiliary thyristors are fired so as to form a conductive path which includes the cathode-gate path of the other power thyristor, the auxiliary thyristors, and the gate-cathode path of the protected thyristor. Normal firing of the power thyristor is achieved via the auxiliary thyristors, which may be fired by light pulses or current pulses. Thus, the auxiliary thyristors perform protective and normal firing operations.