PURPOSE: To enable internal self-diagnosis by simply altering the program of a programmable device by loading the device with the self-diagnostic program stored on a small board.

CONSTITUTION: A connector 8 is arranged other than the connector 2 connected to a system main body to connect a small board 7. The board 7 has an ROM having a down load program mounted thereon and, when a power supply is closed, a programmable logic array 6 is loaded with the program of the ROM through the connector 8. The array 6 becomes a circuit different from a normal circuit by the movement of the loaded program and sends self-diagnosing check signals C1, C2, C3 to respective usually controlled processing blocks 3, 4, 5 to receive the response of the blocks to judge whether circuit operation is normal. By this constitution, the program of the programmable device is simply altered to execute various self-diagnosises.