PURPOSE: To reduce the developing manhour of firmware by securing such as constitution where a CPU performs the inter-processor communication to decide the presence or absence of an I/O (input/output) processor and then carrying out directly an I/O instruction when the absence of the I/O processor is confirmed.

CONSTITUTION: A CPU 1 performs the inter-processor communication against an I/O processor 3 to decide the presence or absence of the processor 3. When the presence of the processor 3 is confirmed, the CPU 1 makes the processor 3 carry out an I/O instruction. While the CPU 1 carries out the I/O instruction by itself when the absence of the processor 3 is confirmed. As a result, the I/O instruction is carried out just with a single type of firmware regardless of the presence or absence of the processor 3. Thus it is possible to reduce the developing manhour of firmware and also to omit the developing manhour for replacement processing.