PURPOSE: To perform the selection of a timepiece unstable in oscillation and the stable detection of oscillation immediately after a power source was closed by detecting the irregularity of an oscillation state, by performing repeated operation for measuring the cycle of a timepiece and comparing measured values by a microcomputer.

CONSTITUTION: A rate measuring device repeatedly performs the cycle measur ing procedure shown by the drawing and measuring cycles are set to t0, t1.... If a state satisfying tn-tn-1<Δt is continuously succeeded m-times (step 31) with respect to a tolerant cycle variation quantity Δt and a max. cycle tmax and a min. cycle tmin satisfy (step 32) tmax-tmin<tw with respect to max. cycle variation quantity tw, this state is judged as oscillation stability not generating even a minute amount of step shift and main measurement is started. The state other than this state is judged as oscillation non-stability (step 35) and, when tn-tn-1<Δt is not continued m-times, the number of measuring upper limit times nmax is provided and, even when n≥nmax was formed, it is judged that oscillation is unstable. By this method, the selection of a timepiece unstable in oscillation and the stability detection of oscillation, immediately after a power source was closed, are performed.