PURPOSE: To detect the opposite phase of a three-phase AC power source assuredly in a simple circuit construction by combining a synchronous circuit, a counter circuit, a pulse signal circuit and a resetting circuit.

CONSTITUTION: When power source lines 2 and 3 of phases S and T are connected in the opposite phase, an output VTR synchronized with the voltage across phases S and T between the power source lines 3 and 1 is issued between output terminals of a synchronous transformer 8, while between output terminals 13 and 14 an output VST synchronized with the voltage across phases S and T between power source lines 2 and 3 is issued. Consequently, the outputs S15 and S16 of zero cross circuits 15 and 16 will be high-levelled corresponding to the positive half wave of signals VTR and VST respectively. In the rise of the signal S16 the output P19 of a one shot multiplexer 19 issues an output, but the output of an AND gate 20 remains low-levelled because of the low level of the signal S15. No counter 18 is therefore reset, but it is counted up by the signal S15 and an abnormal signal is outputted.