PURPOSE: To stably perform a proper digitization operation by providing the following: a comparison means which compares the output voltage of a smoothing means or a holding means with a reference voltage; and a means to control the duty ratio of an output pulse from a comparator to about 50% as required.

CONSTITUTION: The following are provided: smoothing means 10 to 12 which smooth the output signal of a comparator 3; holding means 14, 16, 17 which selectively hold the output voltage of the smoothing means 10 to 12; and a comparison means 19 which compares the output voltage of the smoothing means 10 to 12 or the holding means 14, 16, 17 with a reference voltage. In addition, the following is provided: a means 8 by which an offset correction voltage proportional to a comparison result by the comparison means 19 is added to an input signal on one side of the comparator and which controls the duty ratio of an output pulse from the comparator 3 to about 50% as required according to the data pattern of an optical disk. Thereby, a digitization error caused by the difference between the inner circumferential part and the outer circumferential part of the data pattern or the disk is eliminated, and a digitization operation can be performed stably.