When information is reproduced from an optical disk, a comparator compares a reproduced signal with a slice signal to generate binarized signals, while the slice signal is corrected continuously for external disturbances in a reproduced signal. A switch changes the time constant of the frequency response of the slice signal according to the reproduced signal. For example, the frequency response of the slice signal is set faster in a first region in a prepit area of the disk and in a record area of the disk so that the slice level converges quickly, while the frequency response is changed thereafter to a low time constant for normal signals. Thus, inaccuracy in binarization of the reproduced image signal is minimized, and the sector format is performed efficiently.