PURPOSE: To diagnose the abnormality of a control device and a controlling object with a simple constitution, by reading a signal inputted into the input terminal of a microcomputer at the time of self-diagnosing mode, and sending a signal corresponding to the read signal to the output terminal of the computer.

CONSTITUTION: When a power supply is connected and a mode change-over switch SW1 is turned on, a microcomputer 1 is set to the self-diagnosing mode. Then the computer reads each of the signals inputted into input terminals IN2, IN3,W and sends the signals to output terminals OUT2, OUT3,W under the same level. This mode is maintained until the power supply is stopped. When a abnormality occurs in a device to be controlled, the troubled point can be diagnosed by individually driving plural functioning sections which control each function.