PURPOSE: To use a microprogram for normal processing also to a microprogram for diagnosis by providing a branch address register for diagnosis and a diagnosis mode bit to a microprogram controller.

CONSTITUTION: When a branch command for diagnosis emerges at a hardware control command part, the branch address information for diagnosis of a branch address register 8 for diagnosis is stored to a microinstruction word reading address register 7 via a signal line 53 since a diagnosis mode bit 9 is set to "1". This branch address information is branched into addresses of an after-processing microprogram for diagnosis which is previously stored in a microinstruction word store device 5. If a diagnosis processor 4 receives a report of a normal or abnormal end during said processing, the processor 4 performs the processing in accordance with its received report and sets the next diagnosis start address information to the register 7 via a signal line 52. Thus a microprogram for normal processing is used also to a microprogram for diagnosis by repeating said operations.