1. A storage device using a nonvolatile cache memory, comprising: a host interface with respect to which a command and first data block from a host device dealing with first length as a read/write unit are input/output, a command analyzing section which analyzes contents of the command, a memory interface for the nonvolatile cache memory, a disk interface which inputs/outputs a second data block with respect to a hard disk dealing with second length as a read/write unit, and a read/write processing section which reads out one of partial data and whole data of the second data block having starting end and terminating end addresses which surround starting end and terminating end addresses of the first data block and lie closest to the starting end and terminating end addresses from the hard disk and writes the read data to the nonvolatile cache memory before a data process using the nonvolatile cache memory is performed based on the command.