A cache processing unit manages the data, which is in a cache memory, in a page unit including plurality pieces of block data each of which serves as an access unit of a host, and processes input and output requests from a host to a storage device. In a case in which dirty data in a cache memory which is updated by a host and newer than stored data in a storage device is to be written back to the storage device, and when an incontinuous area is determined in a page of the dirty data, a write-back process unit reads out an incontinuous area from a storage device when a read command is issued, subjects it to a merge so as to generate a continuous area, and performs a write back by one time of write command issue.