PURPOSE: To utilize a system efficiently without charging excessive load to software by providing a channel control device with a means for storing alternative buses to switch an access bus automatically.

CONSTITUTION: When an input/output (I/O) instruction for a data channel #04, an I/O control device a5 and an I/O A6 is outputted from a CPU1 to the channel control device 3 and during the processing, an I/O instruction for an I/O B6 is outputted, the following operation is executed. Since the device 3 is provided with the means storing the alternative buses, the device 3 operates so as to check the existence of the alternative buses. When the buses for a data channel #14 and the I/O control device b5 exist, the buses are used automatically if the buses are not being used to execute the I/O processing of the I/O B6. If said buses are being used, all buses are checked, and only when all buses are being used, the result is reported to the CPU1. Consequently, the efficieny system preventing excessive load to the software can be used.