A standard physical medium 1320 as loaded into library unit 1080 even in the absence of any load request from a host computer 1020 is regarded as a spare physical medium 1330 for use when the standard physical medium 1320 is under malfunction, and such medium is put in a spare media storage space 1340 while eliminating issuance of any notice to the host computer 1020. Whereby, in a computer system with redundancy configuration, it becomes possible to prevent the host computer from making use of any spare physical media.