PURPOSE: To detect a flow rate without distortion even at the time of large flow rate having the large phase modulation degree and to magnify a range ability, by using a comparator having saw-tooth wave characteristics which performs a phase comparison including a duty ratio of pulse.

CONSTITUTION: A phase comparator 7 decides outputs RA, LW by the value of inputs R, L and variables X, Y expressing the state. Saw-tooth wave characteristics in each 2π of a phase difference in V out characteristics in accordance with a phase difference of the inputs R, L is obtained. Therefore, a phase demodulation is performed without distortion to the range of ±π centering the zero radian point of the phase difference.