PURPOSE: To surely apply a tracking by executing a tracking loop closing command to shift from a seek action to a tracking action in a timing with an output from a means to detect the relative speed of a beam and a track.

CONSTITUTION: A timing, by which a tracking loop closing command (g) is outputted to execute the shift from the seek action to the tracking action, is obtained when the conditions of the relative speed of the beam and track are satisfied in addition to position conditions and phase conditions. Namely, when a beam position signal (e) obtained based on a track error signal (a) and a total light quantity signal (c) is inputted to a period detecting circuit 11, the period detecting circuit 11 outputs a period detecting signal e1 at the time of detecting the fact that, for the relative speed of the beam and track, the period attains within a range to stably execute the tracking action by means of a resistance R11 and a capacitor C11. The signal e1 is compared with a goal position attainment signal (f), the tracking loop closing command (g) is outputted, and the tracking action is started.