In creating replicas to data of a storage volume in a storage system, a technique in which labor for issuing instructions from a processor can be reduced and then data consistency of related volumes can be secured is provided. A single replication instruction including designated information is issued for data of a logical unit serving as a copy source and a copy destination from a replication-control program of the processor. A control unit of the storage system possesses a multiple-replica creation program which enables a processing for creating replicas while securing the data consistency when an access such as a update or reference occurs for a group including a plurality of pairs of copy-source volume and copy-destination volume based on reception of the instruction. A save volume is provided to identify and control data save and update state by copy-source save information and copy-destination update information at the shared memory.