Abstract: Recording data is recorded on a CD-ROM so as to satisfy a relationship expressed as W.gtoreq.D and W>m where W is the data length of recording data recorded on the CD-ROM, D is the data length of effective data useful for being accessed (system area, file groups selected when data is edited and retrieval data such as volume descriptor, path table and directory) and m is the maximum recordable data length of a blank disk of a CD-ROM to which data recorded on the CD-ROM is copied. Then, when the relationship between the recording data length W and the effective data length D is expressed by W>D, said recording data contains randomly quasi-data useless for being accessed. Further, a part (e.g., path table) of the retrieval data is arranged at the end of the recording data upon recording. Thus, an illegal copy from the CD-ROM to a blank disk becomes substantially useless, thereby making it possible to effectively protect a copyright of the CD-ROM.