A storage system enables a core storage apparatus to execute processing requiring securing of data consistency, while providing high write performance to a host computer.A storage system includes an edge storage apparatus 20 configured to communicate with a host computer 10 and including a cache memory 25, and a core storage apparatus 30 that communicates with the edge storage apparatus 20 and perform I/O processing on a storage device 39. When receiving a write request from the host computer 10, the edge storage apparatus 20 processes the write request by writeback. When about to execute storage function control processing, on condition that data consistency is be secured, such as pair split processing of a local copy function, the core storage apparatus 30 requests the edge storage apparatus 20 to perform forced destage of dirty data in the cache memory 25 and then executes the storage function control processing after the completion of the forced destage.