PURPOSE: To enable the measuring of a track pitch and the unevenness thereof without affecting the width of a pit by detecting a leakage value of a signal with a specified cycle synchronizing a signal after demodulation based on the reflected light from a pit on an adjacent track leaked into a main beam.

CONSTITUTION: An addition circuit 513 measures a combined light of a 4-split photodetector for a main beam reflected light and peak holding circuits 5331-533n and bottom holding circuits 5341-534n detect variations of the combined light at the upper and lover limits within a specified time. A delay circuit 511 cancels a time difference between preceding and following beams and an A/D conversion circuit 538 detects the size of the reflected light from a pit and a trigger pulse generation circuit 531 generates a trigger pulse from a demodulation signal to an EFM signal. A counter 532 determines a specified time from a trigger pulse reference clock and measures the leakage of a signal by a signal with a specified cycle to determine a track pitch and the unevenness thereof with an arithmetic circuit not illustrated. A control section works to put a main beam at the center of a track thereby letting a sub beam between tracks.