PURPOSE: To automatically diagnose each sensor circuit which is an object in a short time without executing actually a processing operation using each sensor circuit by allowing a microcomputer to execute the processing for a diagnosis and a display in a prescribed sequence.

CONSTITUTION: In accordance with an instruction from the outside, a sensor circuit which is an object of a diagnosis is constituted so that the connecting state or the function state of the sensor circuit is satisfactory or not, and its result can be displayed without operating actually a device for using these sensor circuits. For instance, a CPU 1 executes the data exchange to each sensor circuit such as a thermistor circuit 6, a weight sensor circuit 7, an absolute humidity sensor circuit 8, etc., in accordance with a control program, executes the comparison of a comparison value stored in advance in a RAM 3 and an output value from each sensor circuit and diagnoses whether a connecting state and a circuit function are satisfactory or not, and displays its result on a display part 10. In such a manner, the diagnosis of various sensor circuits can be performed automatically in a short time without executing an actual processing operation.