A bottom hold circuit generates a bottom hold signal (BH signal) from an RFDC signal from an RF amplifier, and sends it to a comparator. The comparator compares a reference level and the BH signal, and generates an Rfdet signal having H level if the BH signal was found to be lower than the reference level. The Rfdet signal is a signal having H level when a laser spot falls in a data-recorded area on an optical disk, and having L level when in a non-recorded area. When it was detected that Rfdet signal had H level throughout a single turn of the disk, a servo-processing microcomputer controls an optical pickup so as to apply tracking servo. This makes it possible to precisely detect a boundary position between the data-recorded area and non-recorded area. If the optical disk is decentered, the boundary area corresponded to the amount of decentering can successfully be isolated from the data-recorded area.