PURPOSE: To determine phase difference highly accurately regardless of input noise by detecting predetermined point of phase for different input signals and then determining phase data of each input signal based on the detected signals and a reference signal having longer period.

CONSTITUTION: First and second input signal detecting sections 10, 11 receive different input signals and detect predetermined point of phase, e.g. zero-cross point, of the input signals to deliver detection signals to a timing generating section 12. The timing generating section 12 generates a data timing signal for acquiring phase data based on the detection signals and a timing signal for acquiring phase data of a reference signal and the phase data are delivered through a data acquiring section to a phase detecting section 14. The phase detecting section 14 creates an amplitude data based on a reference signal having period longer than an input signal from a reference signal generating section 15. Phase data of reference signal, previously stored in a memory, is then selected and inputted to the acquiring section 13 based on each timing signal thus calculating phase difference of input signal based on the phase data.