Timing jitter sequences Δφj[n] and Δφk[n] of respective clock signals under measurement xj(t) and xk(t) are estimated, and a timing difference sequence between those timing jitter sequences is calculated. In addition, initial phase angles φ0j and φ0k of linear instantaneous phases of the xj(t) and xk(t) are estimated, respectively. A sum of a difference between those initial angles and the timing difference sequence is calculated to obtain a clock skew sequence between the xj(t) and xk(t).