PURPOSE: To execute efficiently a diagnosis program by dynamically referring an information register for controlling the diagnosis program to monitor an idle time and control the execution at the execution of the diagnosis.

CONSTITUTION: The diagnosis program is executed from a processor 10 to a channel 11 or 12 and then a disc device 13 or 14 and from a disc driving device 16a in a disc driving device group 16 to a disc driving device 16b through a disc connecting device 15. The register for controlling the execution of the diagnosis program is formed in the disc device 15 to monitor the idle time of the disc control devices 13, 14 and the disc connecting device 15 on the basis of the program. After completing the preparation of diagnosis of all the disc driving devices 16, i.e. setting up all the disc driving devices, said register executes the diagnosis program. Thus, the plural devices can be diagnosed efficently without delaying the execution by a fixed time.