A clock waveform XC(t) is transformed into a complex analytic signal using a Hilbert transformer and an instantaneous phase of this analytic signal is estimated. A linear phase is subtracted from the instantaneous phase to obtain a varying term &Dgr;&phgr;(t). A difference between the maximum value and the minimum value of the varying term &Dgr;&phgr;(t) is obtained as a peak-to-peak jitter, and a root-mean-square of the varying term &Dgr;&phgr;(t) is calculated to obtain an RMS jitter.