The management computer 600 copies data in the volume 111 within the storage device 100A to the volume 115 within the storage device 100B. Upon completion of copy processing, the volume 115 is set to write protect. Then, the data in the volume 111 is read from the storage device 100A, and the data in the volume 115 is also read from the storage device 100B, to compare the two data items. Only when the data items agree with each other, setting of FC switch 400 is changed, and a target of the host computer 500 is switched from the storage device 100A to the storage device 100B.Therefore, it is possible to detect or prevent an illegal data modification in the course of data copying from the source volume to the destination volume.