An apparatus for detecting a faulty switching device of a current-supply type full-bridge inverter includes current transformers connected to each pair of series connected switching devices of the inverter. A differential amplifier is provided for subtracting one detected current value output by one current transformer from another detected current value output by another current transformer. A rectifier is provided for smoothing and rectifying the output of the differential amplifier, and a comparator is provided for comparing the smoothed and rectified output of the rectifier with a reference value.