PURPOSE: To perform fault diagnosis rapidly by providing a fault diagnosing circuit similar to a diagnosed analogue output circuit for the purpose of fault diagnosis and inputting analogue data of the same value to both circuits and comparing analogue valures of both circuits with each other at a diagnosis point.

CONSTITUTION: Data from processing unit 100 such as a computer is converted to an analogue quantity through register 301, D-A converter 401 and insulating circuit 501. For the purpose of performing fault diagnosis of analogue output interface 120 which is outputted as a control quantity to a control object, fault diagnosing analogue output interface 130 similar to this analogue output interface 120 is provided. Fault diagnosing analogue data of the same value is given to circuits 120 and 130, and outputted data are compared with each other at a diagnosis point by comparators 710 and 720; and in case of disagreement, a fault is discriminated.