Provided is a storage system enabling the addition of a storage apparatus without going offline. When a first storage apparatus receives from a host system a command for writing first data in any one of the primary volumes in a primary volume group, it writes the first data in the relevant primary volume and sends to a second storage apparatus a first command for writing the first data in a secondary volume specified using first mapping information. When the second storage apparatus receives from the host system a command for writing second data in any one of the secondary volume groups in a secondary volume group, it writes the second data in the relevant secondary volume and sends to the first storage apparatus a second command for writing the second data in a primary volume specified using second mapping information.