Disclosed herein is a computer system which comprises a plurality of input/output devices, an adapter device coupled to the plurality of input/output devices and a main processing unit for supplying an input/output request to each of the plurality of input/output devices through the adapter device. A plurality of input/output commands are respectively issued to the plurality of input/output devices from the main processing unit as one input/output start command. The adapter device specifies to which input/output devices the plurality of input/output commands included in the one input/output start command correspond. Further, the adapter device supplies input/output commands to the specified input/output devices respectively.