PURPOSE: To retry a command at a high speed by storing the command that is carried out at the immediately preceding time point and reading this stored command out when the same command is retried.

CONSTITUTION: An input/output controller 10 contains a command memory buffer 12 which stores a command that is carried out at the immediately preceding time point and a retry means which reads the command out of the buffer 12 and supplies it to an input/output device control circuit 15 in case the same command is retried based on the command executing result. The retry means consists of a switch circuit 13 and a switch circuit control part 15' of the circuit 15. Thus it is not required to receive a command again from a channel device when the command is retried. As a result, a high-speed operation is kept for an input/output device.