PURPOSE: To easily separate a fault generating source even when data are transferred toward an input/output device from a channel, by fetching and checking the data sent to a bus from the channel also at the channel.

CONSTITUTION: When data are transferred toward an input/output device from a channel device 10, these data are temporarily stored in a data-out register 11. The output data of the register 11 is sent to a bidirectional data bus 30 via a driver 12 and then fetched and stored temporarily by a data-in register 14 via a receiver 13. The output data of both registers 11 and 14 undergo the check through a noncoincidence detecting circuit 15. When the noncoincidence is detected, a fact that an error occurred is displayed. Thus a fault generating source is surely detected.