A main controller of a recording medium dualization system reads, when the power is turned on, three types of information: USE information (status information of a recording medium of each medium board) indicating whether or not the medium can be used; ACT information indicating which of the recording mediums is or was used in an active system; and PWON information indicating whether or not the power for the recording medium is turned on, whereupon the main controller sets the information to denote that the master system is an active system and executes a mirroring process whereby the USE information and the PWON information of the slave system are identical to the master system. When it is found that the recording medium of the master system is removed, on the basis of the USE and ACT information, the slave system is switched to an active system, and the corresponding information setting is performed. When it is found that the recording medium of the master system is mounted, on the basis of the PWON information, the USE information and the PWON information of the slave system are mirrored onto the master system.