To sense surely a fault of unverters with a parallel connection structure, by providing a low-pass filter whereto the output of a computer for the cross current of the inverters is inputted so that it feeds a fault sensing level top a competitor.

The respective output currents of inverters 11, 12, 13 which are sensed respectively by current sensors 31, 32, 33 are inputted to a cross-current sensing circuit 41 to sense three-phase cross currents I41U-I41W of the inverter 11. Then, summing the cross currents I41U-I41W, the absolute value of the summed current is computed by a computer 51 to give a sensing level FL51' in its output. Further, the sensing level FL51' is inputted to a low-pass filter 71 to output a sensing level FL71 to a comparator 61'. The conspirator 61' comports the sensing level FL71 with a set level FL* to output a fault signal Sf'. Therefore, even in the case of the short-circuit fault of a dummy resistor, the fault can be sensed surely without raising the set level FL* for the fault signal Sf'.