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.