When data are to be recorded on a recording medium having imparted thereto a medium ID that is identification information unique to the recording medium, an additional ID related to the medium ID is generated on the basis of the medium ID. Then, the data are encrypted and recorded on the medium along with the additional ID. In reproduction, the medium ID and the additional ID attached to the data are read out from the medium, and then compared to determine whether both of the medium ID and additional ID meet a predetermined matching condition. When the additional ID attached to the data has been determined as meeting the predetermined matching condition, the data is judged to be reproducible data. In this manner, it is possible to promptly determined whether or not the data recorded on the medium are fairly-recorded data, which can effectively prevent unfair use of the data.