PURPOSE: To permit an input/output means used in a regular diagnosis to be selected, by making the input/output device hold a substituting function at the time of executing diagnosis, and diagnosing an available input/output means without using specific preliminary diagnosis.

CONSTITUTION: An input operation for preliminary diagnosis is stored in advance in a third memory 2c in a main memory 2, and the preliminary diagnosis is performed at the beginning of the regular diagnosis, by using the memory 2c at the time of starting up a system. A preliminary diagnosing means IN8 diagnoses the available input/output means, and stores a result in a holding means 23. A selection means 24 selects and switches to the input/output means used in the diagnosis from the result. The above switching is performed by setting the forefront address of an input/output routine corresponding to the input/ output means at the input/output pointer of the selection means. A diagnosing means 21 performs the diagnosis by using the input/output means set at the input pointer, and the output pointer.