PURPOSE: To shorten the time required for test of a microcomputer during production and to simplify an external test equipment required for test to perform the trouble self-diagnosis of the microcomputer after completion.

CONSTITUTION: The microcomputer is internally provided with a D/A converter 6, an H/L comparator 10, an B/L flag 13, various setting registers, a connection switch 12 to external terminals, and a test ROM 15. The D/A converter 6 and the H/L comparator 10 can be connected to external terminals of the microcomputer by the connection switch 12. Only when the test mode is set from the external, the test ROM 15 is operated to control these devices and each function of the microcomputer. Various tests are performed by contents of the test ROM 15.