PURPOSE: To detect and control accurate phase angles of two sinewave AC quantity by measuring directly and using instantaneous values of both waveforms at an intersected point with the waveform at one side of switching contacts and the polarity reversal waveform at another side.

CONSTITUTION: DC bias portion is obtained from the average value of the instantaneous values at more than two intersected points which are continued with the sinewave AC quantity at one side and the reversal wave for its zero potential at another side, and the instantaneous values of both AC currents are calculated from the difference between measured values at each intersected point and the bias portion. Next, wave height values are obtained from the instantaneous values and the measured values for the AC quantity at the central points of each intersected point. Further, the phase angles of both AC quantity are detected from the wave height values and the instantaneous values. Then, the wave height values, angular frequencies, phases, etc., are analyzed from these locuses of the intersected points to control the phase angles of both AC quantity. The phase angle detection can be thereby performed in high accuracy, and also various kinds of systematic trouble caused by an overcurrent can be prevented.