To decode copyright information recorded based on a very small change in the edge position of a pit within a short time by reducing the number of integrating times.

An intercode interference amount is calculated according to a pit pattern input from a Viterbi decoder 5 and recorded in an optical disk 1, and the intercode interference amount is eliminated from a reproducing signal obtained from an equalizer circuit 4. As a copyright information detector 9 decodes copyright information from the reproducing signal reduced in noise (intercode interference amount is regarded as noise), the number of integration samples necessary for decoding is reduced, and the copyright information is decoded within a short time.