To provide a new disk cache system having a fast accessible semiconductor disk device and a magnetic disk device with large storage capacity combined.

When a series of read requests to the magnetic disk device 12 is generated, data read out of the magnetic disk device 12 is passed to an OS and application, and also stored in the semiconductor disk device 11. When another series of read requests to the magnetic disk device 12 is generated, data are read out of the semiconductor disk device 11 on condition that the same data are present on the semiconductor disk device 11, so a fast read becomes possible. Cache data stored in the semiconductor disk device 11 are effective even after the power source is turned off and on. Therefore, even when the system power source is turned off, the contents in the disk cache which are structured so far and high in hit rate can effectively be maintained.