To detect the fault of a detector even if various detectors break down and the fault detector cannot detect the fault by judging the fault of the detector when an estimation signal on a state signal which the detector detects is operated and a difference occurs between the estimation signal and the detection result of the detector.

A second signal whose relation with a first state signal detected by a detector being the object of fault detection is clear is selected and an estimation signal on the first state signal is operated from the second signal. When a difference occurs between the estimation signal and the detection result of the detector, the fault of the detector is judged. The change of the DC voltage Vd of a D-C capacitor is estimated from a converter input current Ir being a cause for deciding the voltage of a DC capacitor 4 and from power voltage. When the difference occurs between the estimated value and the change quantity of the voltage detection signal Vf of a voltage detector 5 occurs, the voltage detector 5 is judged to break down.