PURPOSE: To decide the phase of an input signal without being affected by its waveform distortion by deciding that the phase is advanced when the value found by an arithmetic means is negative or delayed when positive.

CONSTITUTION: A 1st analog AC signal I1 and a 2nd analog AC signal I2 are inputted first and converted into digital data by a 1st A/D converter 11 and a 2nd A/D converter 12. The two digital data are written in two storage circuits 311 and 312 with clock signals synchronized with clock signals supplied to the two A/D converters 11 and 12. The digital data written in the storage circuits 311 and 312 are read out with clock signals which are different in time corresponding to an electric angle π/2 and supplied to an arithmetic part 314. The arithmetic part 314 multiplies the two supplied digital data to decide whether the arithmetic result is positive or negative and then decide that the phase is advanced when the result is negative or delayed when positive, thus deciding the phase without being affected by the waveform distortion of the input signal.