PURPOSE: To effectively detect a malfunction when an operation in which a firing angle is about 90 degrees, is continued.

CONSTITUTION: LEDs 6, 7 for detecting a forward voltage and a reverse voltage of a thyristor valve, and a detector 12 for detecting that a voltage applying period of the bulb 1 is a predetermined period or longer in one cycle according to the outputs of voltage detectors of the LEDs 6, 7, are provided. Further, an AND circuit 16 which outputs a signal when both a signal indicating that the valve 1 is operating and a signal from a time detector are input, and a time detector 18 which outputs a voltage malfunction signal when the output of the AND circuit 16 is continued for a predetermined period in each cycle, are provided.