PURPOSE: To eliminate the destruction of fault information and an adverse influence on the other units by providing a field storing information obtained by means of encoding a suspected unit for a part of the field of a microinstruction.

CONSTITUTION: The suspected units 1-2 to 1-4 can be specified to a certain degree in correspondence with the content of a microprogram in the suspected field 2-5 of the microinstruction. Thus, data is transferred between the diagnosed units 1-2 to 1-4. In the case of the microprogram in the diagnosed unit 1-2, the codes for indicating the diagnosed unit 1-3 as the first suspected unit and the diagnosed unit 1-2 as the second suspected unit can be described at the time of design in the suspected unit code field 2-5 of respective steps in a processing routine executing the processing based on data received from the diagnosed unit 1-3. Thus, the destruction of information to be extracted and the adverse influence on the other units which normally operate can be prevented.