here a session is information in which the source and the destination are related fig4 illustrates a session control table each cm stores a session control table for example an administrator sets a combination between volumes in the source and the destination before starting copying each cm retains information of a combination in the memory thereof as a session control table the session control table contains information regarding a copying session and therefore exists for each session the information regarding a copying session includes a session id session type source volume destination volume copying range and storage cm the session id is unique identification information that exists for each session the session type is information indicating a copying type such as remote copying or local copying the source volume is information including a volume number of the source and an initial logical block address lba of the source the destination volume is information including a volume number of the destination and an initial logical block address of the destination the copying range is information indicating the range of the copying process for example the range of the copying process is indicated by the number of blocks the storage cm is information for.