PURPOSE: To improve accuracy of an operating time by detecting rise and fall points of rectangular wave pulse signal upon receipt of the rectangular wave pulse signal and detecting phases of the signals inputted in accordance with the time when the above detections are made.

CONSTITUTION: In phase detecting circuits 2, 3, when a zero phase current or zero phase voltage is inputted, they are detected for every zero cross point of the AC signal, i.e. every time when positive zero cross points and negative zero cross points are appeared, and the waveforms are reversed in phase pulse generating circuits 27, 37 to output the rectangular wave pulse signal. Then, the phase detection is made by the phase detecting circuits 2, 3 based on the rise and fall points of this rectangular wave pulse, and the phase difference is calculated by a CPU 4.