PURPOSE: To simplify the microprogram in a diagnostic routine and to reduce the capacity of the microprogram by simultaneously and continuously diagnosing plural RAG functions.

CONSTITUTION: At the time of RAS function diagnosis, an error branch signal 204 inputted to a microprogram address setting part 13 is kept inactive by the action of an AND gate 17 because of an active test mode signal 202 though an error detection signal 201 of a RAG function part 16 is active. Therefore, the microprogram address setting part 13 indicates the next incremented address without branching to an error processing routine.