To make the mirroring process of storage devices effective without any waste and shorten its processing time by performing the mirroring process (copying process) for stored information in specific units between storage devices when there is no operation instruction to the storage device.

On a CPU board 10, a CPU 11 which drives and controls respective devices, a DRAM(dynamic RAM) 12, a SRAM(static RAM) 13 wherein stored data are nonvolatile, an IPL(initial program loader) 14 stored with a boot program, a storage controller 15, and an input controller 16 are mounted. Then a mirroring table stored in a multiplied information table in the SRAM 13 contains data to be mirrored so that the data are divided into various constant numbers of blocks of, for example, several K bytes or several M bytes and the mirroring states (copy state) of the respective blocks are stored. When there is no operation instruction to the storage devices, the mirroring process (copying process) is performed for the stored information in specific units between the storage devices.