PURPOSE: To prevent the abnormality of plant operation caused by a defective sensor by always detecting abnormal signs of respective differential pressure sensors or respective pressure sensors mounted inside a process instrumentation rack.

CONSTITUTION: A central processing circuit 31 takes in a measurement data composed of respective detection signals and respective test detection signals from a pressure measuring part 5 and an environment data from an environment measuring part 6, and computes the transitive data and its change rate, then the data is stored in a data storage circuit 32. When the present transitive data or its change rate satisfies the specified conditions for judging abnormality in comparison with the past transitive data or the change rate thereof, respective differential pressure sensors 15 or respective pressure sensors 18 constituting the part 5 is judged to be defective.