To perform phase detection with high accuracy by avoiding dispersion in detecting phase signals and degradation in accuracy caused by acceleration of a rotating body.

This phase detector for rotating body comprises a phase signal generating means for outputting phase signals θa indicating the phase of a rotating body at prescribed phase intervals, counter for counting time, time interval measuring means for measuring time intervals ts between events of a plurality of phase signals, correction value determining means for computing a correction amount tm for time interval measurements, time interval correcting means for computing a time interval correction value tc based on the correction amount, speed computing means for computing a rotation angular speed V based on the time interval correction value, and phase-computing means for computing phases θ, based on the rotation angular speed, the latest phase signal and the time measurements. Adverse effects caused by changes and the like in the time interval by acceleration can be estimated and corrected by comparing and correcting the time interval measurements and the previous time interval correction value, so that computation accuracy of phase detection can be enhanced.