PURPOSE: To carry out the operation check surely in a desired timing without changing the program in a controller body by transferring the necessary diagnosis program, only in the case where the diagnosis operation is desired, and executing the operation check of a controller for vehicle.

CONSTITUTION: A controller 2 as the diagnosis object consists of an input part 3, output part 4, and a microcomputer 5 arranged between both 3 and 4. An external circuit 20 installed between the input part 3 and output part 4 diagnoses the operation state of the controller 2 through the check of the response characteristic between the input and output. In this case, if a switch 13 is closed, a diagnosis program which is previously stored in the memory 29 in a microcomputer 28 is transported to the program area A in the RAM 10 of the microcomputer 5, and when the control program is executed, the necessary diagnosis is executed by calling the contents of the program area A. Accordingly, even if the operation state of the controller 2 changes, the execution of the diagnosis operation can be surely prevented, if the diagnosis operation is not desired.