PURPOSE: To secure the detection of an open-phase, by monitoring the voltage across both the terminals of a filter capacitor, and by judging the open-phase while extracting the voltage across both the terminals at the sampling period equal to a half period of the frequency of AC current.

CONSTITUTION: Both the terminals of a filter capacitor 4 are connected respectively with the input terminals of an inverter 5. The voltage across both the terminals of the filter capacitor 4 is sensed by a voltage sensor 7, and is extracted by a sampling circuit 8 at a half period of the frequency of AC current flowing through a stringing. Subsequently, the extracted voltage is inputted in succession to a differentiation computing circuit 9, an absolute-value computing circuit 10 and a first-order delay circuit 11, and thereby, the accumulated value of the changing quantity of the voltage across both the terminals of the filter capacitor 4 is obtained. The accumulated value is inputted to a controller 12, and is compared with a predetermined reference value. When the wherein the accumulated value continues to be judged as an abnormal value exceeds a predetermined reference time, a protective-operation command is outputted from the controller 12 to the inverter 5.