Data transfer is performed to and from a host computer using a first block as the minimum unit. Data transfer is performed to and from a storage area using a second block as the minimum unit. A second block set of the storage area stores data obtained from performing data conversion processes that change the size of the data itself, with a first block set as the unit. Here a correspondence relationship is generated between the first block set and the second block set. In response to a read request from the host computer, a second block set, which corresponds to the first block set that includes the first block that is requested, is read, a reverse-conversion process is performed, and the data is sent to the host computer.