PURPOSE: To enable accurate frequency measurement, by providing a phase detector with a phase deviation compensating circuit.

CONSTITUTION: A phase detector 31 of a frequency coarse measurement system 32 is provided with a phase deviation compensating circuit 33 to make fixed phase deviations present in a frequency precise measurement system 18 and the coarse measurement system 32 coincide with each other. Then, the circuit 33 calculates sin and cos of a phase value to be compensated beforehand to set a voltage division ratios of variable resistances 25W28, difference thereof is determined with differential amplifiers 29 and 30 and a phase angle as output thereof is turned by a specified value to compensate the phase. Then, a fixed phase deviation of the coarse measurement system 32 is made to coincide with a fixed phase deviation of the precise measurement system 18 thereby enabling accurate measurement of frequency.