PURPOSE: To achieve a higher accuracy by performing a measurement by a time counting clock signal with a time difference being not synchronous in frequency during a change in polarity to make an averaging with a lowpass filter having a specified time constant.

CONSTITUTION: As a time constant the reciprocal of frequency difference be tween n×f1, and fCLK is employed, where n is an integer, f1 is frequency of first signal oscillated by oscillator 2, fCLK is frequency of oscillator 6, and n is selected so that n×f1 is closest to fCLK. Counts counted with a time counter 1 is added cumulatively with a lowpass filter 5 to average. This enables measure ment of a phase difference at a high accuracy.