When an ECU applies to an igniter a plurality of pulse signals, the duty ratio of which is lower than a predetermined duty ratio, as a lock release signal, a latch circuit of a lock inhibition circuit stops applying a shut-off signal to a shut-off circuit thereby to release a lock condition of an IGBT. A drive main circuit thus restarts current supply to the IGBT. A condition detection circuit outputs to the ECU a condition signal corresponding to the current supplied while the IGBT is turned on.