PURPOSE: To expand the limit of high densification and simultaneously to improve reproducing signals in high density recording by making the phase difference of information pits that are adjacent in front and rear as (2N/3)π (where N is an integer).

CONSTITUTION: By modulating the strength of exposing laser, the pits different in depth by which the phase difference in (2N/3)π is given are alternately formed. In this case, since the phase difference also exists between the adjacent pits 2, 3, separation ability is improved by making the reflected waves from the adjacent pits mutually interfere and cancell, and therefore the limit of high densification is expanded. Meanwhile, the shortest pit length or the length between the pits is shrunken 2/3 to 1/2, and the reproduced signals in the high density recording is improved.