PURPOSE: To prevent the burning of a suppression resistor from being generated, by drive-stopping an inverter circuit at the time of a contact drop-out state.

CONSTITUTION: When the contact 3 of a contactor 3a drops out during the operation of an inverter, then current flowing through the contact 3 flows through a suppression resistor 2, and the terminal voltage of the suppression resistor 2 is heightened. To the light generating section of a light insulation element 11, current flows, and light is generated, and by the light, to the light receiving section of the light insulation element, light current flows, and the input as contact drop-out signal to a hold circuit 10 is provided. The input of the output signal of the hold circuit 10 to a base driving circuit 8 is provided, and control signal from a controlling circuit 7 is interrupted in the base driving circuit 8. When an input power source is on, then on this timing because great current flows to a suppressing circuit 2 when the input power source is on, interlocking function is performed by an interlock circuit 9 so that the hold circuit 10 cannot be worked even if the light insulation element 11 is worked. In this manner, the relay contact drop-out is detected and the inverter is stopped, and so the damage and spark of an equipment are prevented from being generated.