A storage system includes a storage device that stores data, a cache memory that caches the data, an information storage unit that stores data configuration information indicating a configuration of the data and state information indicating a cache state of the data in the cache memory, a candidate data selection unit, a first determining unit and a data-to-be-written unit. The candidate data selection unit selects, according to the state information candidate data from the data cached in the cache memory. The first determination unit determines, according to the data configuration information, whether data relating to the candidate data is cached in the cache memory. The data-to-be-written selection unit selects, according to the determination made by the first determination unit, data to be written into the storage device, from the data cached in the cache memory.