PURPOSE: To reduce a burden of a host device side by providing a compression/ expansion processor in the auxiliary storage device.

CONSTITUTION: When a write instruction of data to an auxiliary storage device 20 is executed by an application program, and a file name of its data is given, a CPU 101 of a host device 100 transfers data generated on a main memory 102 to the auxiliary storage device 200 by using an OS program. In the auxiliary storage device 200, data of a data register 212 is compressed by a compression/ expansion processor 220 under the control of a local processor 250, its compressed information is written in a storage medium, the capacity of compressed file information is counted by a counter 221, and a counting value of the counter 221 is written in a real write size register 214. Subsequently, the CPU 101 reads out the compressed file capacity information of the real write size register 214, and executes updating of a directory file of the main memory 102 and the auxiliary storage device 200.