A failure detection device detects the voltage across the main electrodes of an IGBT via a diode. The failure detection device determines occurrence of short-circuit failure in the IGBT when the anode voltage of the diode is lower than a first predetermined reference voltage. Determination can be made, excluding the case of a proper operation corresponding to a flywheel diode in an ON state, preferably together with the condition that the anode voltage of the diode is higher than a second predetermined reference voltage.