To precisely measure a phase fluctuation amount even if a frequency difference between an input signal and a reference signal is large by constituting a frequency conversion circuit by a PLL circuit limited on a prescribed frequency band as the carrier frequency of the frequency-converted input signal and the reference signal is centered.

An input signal and a reference signal are frequency-converted by first and second frequency conversion circuits 1, 2 constituted by a PLL circuit and limited on a prescribed frequency band (for instance, 10Hz) as the carrier frequency is centered at the time. Thereby LPF 4 connected to a phase detector 3 eliminates the carrier frequency of the input signal included in a signal and the reference signal, in order to obtain a desired phase fluctuation component (wonder component), it is higher than the frequency difference of the input signal and the reference signal, and has a higher shield frequency than a prescribed frequency (phase fluctuation component). Thus, even if the frequency difference between the input signal and the reference signal is large (about 10Hz), the phase fluctuation can be measured.