PURPOSE: To protect surely a switching element at short-circuit and also to decrease the residual voltage at operation by providing a thyristor triggered by the secondary output of a pulse transformer and resetting the operation of a detection circuit section.

CONSTITUTION: When a prescribed overcurrent flows, a pulse is transmitted from a pulse transformer secondary side 8b immediately, the operation of a sensor circuit 11 is reset by a thyristor 20 and also a triac 5 is triggered, then the short- circuit current flows to the triac 5 and does not flows to a thyristor 9 being an output switching element so as to protect the thyristor 9. Although the short- circuit current flowing to the triac 5 is limited by a current limit resistor R3, since the surge current rating of the triac 5 is larger than that of the thyristor 20, the resistance value of the resistor R3 is decreased. Then the residual voltage produced by the resistor R3 at normal operation is decreased.