A storage system has a cache memory, a side-file being a storage area prepared in memory, and a copy source storage device. Each time write data is received, the storage system writes the write data to the copy source storage device via the cache memory, and also writes the received write data to the side-file. The storage system sends the plurality of write data accumulated in the side-file to another storage system at a certain timing. Thus, the plurality of write data stored in the copy source storage device is written to the copy destination storage device in the other storage system.