An external storage device 110 includes a semiconductor memory 200, a hard disk 210 having an access speed lower than that of the semiconductor memory 200, a judgment part 275 for determining whether or not data can be stored in the semiconductor memory 200, and an access processing part 277 for compressing and storing data into the semiconductor memory 200 after it is determined that the data can be stored in the semiconductor memory 200, and for storing the write data into the hard disk 210 when it is determined that the data cannot be stored in the semiconductor memory 200. The storage capacity of the external storage device 110 is larger than the storage capacity of the semiconductor memory 200.