PURPOSE: To rapidly protect each GAO against a malfunction by providing a plurality of voltage detectors for detecting the malfunctions of the GATs on the basis of an impedance between its gate and its cathode, a signal processor and a display unit.
CONSTITUTION: A detector circuit for a malfunction element is composed of a GTO 11 for forming a switching element group, a voltage detector 12 for individually detecting the malfunctions of the GTOs 11, a current limiting resistor 14, a light emitting diode 15 as a display unit, a transistor 16, a diode 17, and a logic circuit 18 as a signal processor. Thus, when one of the GTOs 11 malfunctions, the impedance between its gate and its cathode becomes low. Accordingly, its voltage change is detected by the detector 12, which generates a signal. Since the corresponding diode 15 emits a light by this signal, the malfunction GTO can be detected without disconnecting a GTO 11 from a main circuit.