PURPOSE: To protect an electric machine connected with a power supply against damage or failure by determining the phase difference between first and second pulses having different pulse width, deciding whether the phase difference comes within a predetermined range thereby detecting erroneous wiring of power supply reliably regardless of artificial miss or the like.

CONSTITUTION: A pulse generator 2 generates a pulse A between two phases (e.g. phases S and T) of a three-phase AC input power supply 1 and a pulse B between two different phases (e.g. phases T and R). A microcomputer 3 determines the phase difference between the first and second pulses A, B and makes a decision whether the phase difference comes within a predetermined range thus determining erroneous wiring. Since the phase difference is 120° in the case of a symmetric AC three-phase power supply and the phase difference is varied if the power supply is wired erroneously, the erroneous wiring can be determined by verifying the phase difference.