Abstract: The logic blocks which were swept out from a non-compression cache memory are compressed by a compressing circuit and are combined to a compression group (logic sector size) of a fixed length in which a plurality of compression data are collected by a compression group forming unit. The compression group is stored into a logic sector of a disk drive through a compression cache memory. A correspondence relation with a plurality of logic blocks included in the logic sectors of a disk is registered in a mapping table. In response to a request by the logic block number from the host computer, the logic sector (compression group) of the disk drive is read and written with reference to the mapping table.