PURPOSE: To report correct fault contents to a host device, and to shorten the recovery time of a fault controller, by setting up a bus between respective controllers through a microprocessor, and analyzing and editing the fault contents through the processor.

CONSTITUTION: Controllers 3 and 33 are connected to the CPU1 of a file sub- system through channels 2 and 22 respectively, and a microprocessor 6 is connected as an interface between both devices 3 and 33. The processor 6 sets up a bus for the interface to analyze and edit fault contents of the controllers 3 and 33, and report correct fault contents to the CPU1 through the channels 2 and 22. As well as this fault reporting, a substitute bus for the operating system OR of the faulty controller 3 or 33 is set up to shorten fault recovery time, thereby improving the reliability of the system.