In a complex system including; one or more storage systems including a cache and a storage controller; and one or more storage boxes including a storage medium, the storage box generates redundant data from write data received from a server, and writes the write data and the redundant data to the storage medium. The storage box transmits the write data to the storage system when it is difficult to generate the redundant data or it is difficult to write the write data and the redundant data to the storage medium. The storage system stores the received write data in the cache.