To find the occurrence of illegal data and a disk failure at an early stage and to reliably store even data that are not read for a long time.

This device is provided with a cache memory 6, a data dividing part 3 which divides data transmitted from a host device 1 in accordance with the number of physical disks and also generates the parity data of the divided division data, a data transferring part 4 which respectively stores the division data and the parity data into physical disks, a data comparing part 8 which reads the data from the physical disk after storing the division data and the parity data in each physical disk and compares the data with corresponding data stored in the cache memory, and a failure deciding part 10 which decides that a failure takes place in the physical disk storing an uncoincidental part when any uncoincidental part exists in the part 8.