The storage system is configured to expand from small to large configurations at a reasonable cost with performance decided approximately to a target system scale. The storage system has a plurality of protocol transformation units, each used for transforming a protocol to be exchanged with the server or disk drives, and a plurality of data caching control units, each having a cache memory unit for storing data and a function for controlling the cache memory unit. The plurality of data cashing control units are divided into a plurality of control clusters while the cache memory unit is controlled only in its corresponding control cluster independently and the system management unit is connected to an interconnection, such that the system management unit manages the plurality of protocol transformation units and the plurality of control clusters are managed integrally as one system.