PURPOSE: To obtain high reliability by comparing an output potential of peak hold means and a specified potential and outputting the output for discriminating the polarity of a load current of transistor.

CONSTITUTION: A DC potential is impressed between both sides of power transistor modules 1U, 1V, 1W. On the other hand, the difference in forward voltages of reflux diodes 3U2, 3V2, 3W2 is generated on the base potentials of power transistors 2U2, 2V2, 2W2 during the ON/OFF state. So, for instance, when the base potential of transistor 2U2 is received by the peak hold means consisting of an operational amplifier 10, etc., a waveform with the envelope of base potential taken out is obtained as the output of peak hold means. The potential subjected to peak holding is compared in a comparator 14 with the specified potential obtained by resistors 15, 16, then a current polarity signal S1 of phase U is obtained. Similarly, signals S2, S3 are obtained from the base potentials of transistors 2V2, 2W2. The polarity change of output currents for each phase of U,V,W can be learned from the rises of these signals S1-S3.