PURPOSE: To obtain a reproducing system suitable for recording a mark edge of a phase changing optical disk.

CONSTITUTION: An optical phase difference between a recording mark 201 and an erasing part 202 is used for reproducing a signal. Outputs of a reproducing detector 1 bisected in the track direction are respectively inputted to a differential amplifier 2 to take a difference between the two outputs. At this time, a regenerative signal to become a max. value or a min. value at the part of the recording mark edge is obtained. This regenerative signal becomes the max. when a phase difference between the recording mark 201 and the erasing part 202 is π/2, and this peak is obtained when the center of a reproducing laser beam is positioned at the recording mark edge.