PURPOSE: To make possible the detection of effective current without using any insulation means by detecting output currents in an inverter for electric power, etc. and performing the operation with control part output voltages.

CONSTITUTION: An inverter 1 is constituted by plural transistors TR as its main switching elements. The pairing TRs are complementarily controlled to conduction and inhibit states by the respective phase voltages E of a control part 2 and output 3 phase powers U, V, W. The respective currents I in these 3-phase powers are detected and are respectively multiplied 4 by the voltages of the control part 2 to detect the positive and negative of instantaneous currents. Then, the sums 51 in the positive direction in the respective currents are taken and the outputs of the sums and the sums of the detecting currents in the negative direction are added 52, whereby the DC current of the inverter 1 is detected. This DC current is proportional to effective current, whereby the detection of the effective current may be accomplished.