PURPOSE: To decrease in running numbers of input/output instruction in order to look for an idle channel unit by a central processing unit by notifying information whether or not a channel unit is now busy to the central processing unit with respect to plural channel devices.

CONSTITUTION: Channel devices A2 and B2 are connected to under the central processing unit 1. Channel devices A2 and B2 are connected to each other by the communication line. The channel device A2 can find the state by information which is sent through the communication line, and the channel device B2 can find the state of the channel device A2 as well. As a result, when an input/output instruction is executed from the central processing unit 1 to the channel device A2 during use of the channel device A2, the channel device A2 can report the state of the channel device B2 together with its reports in use.