To detect insulation deterioration between a driving circuit and a body chassis regardless of the state of a switching element constituting a switching inverter.

When insulation deterioration is not generated between the driving circuit 2 and the body chassis B, a current hardly flows therebetween, and the amplitude of a voltage at the left end of a capacitor 33 is larger than a threshold Vm. On the other hand, when insulation deterioration is generated, a current flows therebetween, to generate a voltage drop in a resistor 32, and the amplitude of the voltage at the left end of the capacitor 33 becomes smaller than a threshold Vm. A constant positive potential is applied to the right end of the capacitor 33 from a constant voltage source 34, and the potential of an anode terminal of a reflux diode 22a always has a higher potential than a rising voltage of the reflux diode 22a, based on the potential of a cathode terminal.