An optical disk of the present invention has one or more recording tracks formed concentrically or spirally on the disk to which data signals are recorded by forming different-length marks and spaces on the tracks. An ID signal combining marks and spaces of two or more predetermined lengths approximately equal to the length of the marks and spaces forming the data signals is recorded to the recording tracks within a predetermined disk area, thus creating the ID signal by aligning the marks and spaces in the radial direction and recording the aligned marks and spaces to adjacent tracks in a predetermined radial disk area. Also, a reproduction apparatus for reproducing signals from the optical disk is disclosed.