A recording medium as a physically rewritable disk is appropriately used as a write-once type. In order to use the rewritable media in write-once applications, the recording medium in the data structure thereof includes, in a second data unit (an ECC block of 64 Kbytes), flag information WF1-WF4 indicating whether a first data unit (2 Kbytes) is stored. More specifically, whether a plurality of first data units (a sector handled by a host) has been recorded in the second data unit (a recording unit sector on the recording medium) is directly indicated. When an access request is issued in emulation in a disk drive device, the flag information is referenced, and the disk drive device determines whether one of writing or reading of the first data unit as a target responsive to a command is enabled.