PURPOSE: To shorten the elapsed time till finding an abnormal reproduction by performing a head frame detecting operation for every frame.

CONSTITUTION: When an AM is detected in a frame and no error is detected in the CRC verification of ID, that frame is judged as a head frame. When any one condition (condition (4)) is satisfied in a frame expected as a head frame or more conditions are detected for an unexpected frame than for an expected frame, i.e., the AM could be detected under severer detecting conditions, that frame is judged as a head frame and the phase of timing pulse is modified while expecting arrival of a head frame. Consequently, the elapsed time until finding an abnormal reproduction can be shortened even if the interval of SYNC is substantially invariant although an abnormal data reproduction, e.g. an abrupt track jump, takes place.