A disc key detecting unit 3 detects a disc key on the basis of data which is supplied from an RF processor 1. If the disc key can be detected, a system controller 7 controls an optical pickup or the like and reproduces data of a few sectors included in an audio block portion together with data in a TOC portion. On the basis of the disc key, a data decrypting unit 4 decrypts the data in the TOC portion which is supplied from a data demodulating unit 2 and the data of a few sectors and supplies them to a data processing unit 5. The data processing unit supplies the data in the TOC portion and data in a header portion included in the sector to the system controller 7. A validity discriminating unit 8 confirms continuity of time data included in the header portion supplied from the data processing unit 5. If there is no continuity in the time data, a reproducing process or the like is suppressed.