PURPOSE: To restore data of a disk, where any fault occurs, without changing the operation of a central processing unit by performing the data restoration processing at the time of absence of instructions from the central processing unit.

CONSTITUTION: At the time of absence of instructions from the central processing unit, a restoration data read instructing part 104 instructs an R/W control part 101 to read in data of disks other than an unaccessable disk in a disk group 112 in order. Read data is stored in a first buffer part 105, and a data restoring part 106 restores data of the unaccessable disk from data stored in the first buffer part 105 and stores restored data in a second buffer part 107. A restored data write instructing part 108 instructs write of data from the second buffer part 107 to a spare disk at the time of absence of instructions from the central processing unit. After the write, switching to the spare disk is performed by a device switching part 111.