PURPOSE: To exactly identify an investigation item by inferring a plant element which becomes the cause for generating abnormality of a plant or does not become the cause by a cause diagnostic knowledge storage means in which fragmentary knowledge of the cause for generating abnormality is stored, and a non-cause diagnostic knowledge storage means, and displaying each element based on the result of this inference.

CONSTITUTION: A cause diagnostic knowledge storage device 10 stores a series of knowledge information required for searching the cause of abnormality of a plant. A non-cause diagnostic knowledge storage device 12 stores knowledge information required for deciding an equipment which does not become the cause of abnormality. An inferring device 14 infers an equipment and a system which become the cause of abnormality of the plant or do not become the cause, and the equipment and the system about which whether they become the cause or not is unknown, based on various knowledge information and equipment information stored in a non-cause diagnostic knowledge storage device 12 and an interactive equipment information storage device 11. The result of its inference is stored in the interactive equipment information storage device 12, and displayed on an interactive display device 15. In such a manner, an item to be investigated can be identified exactly.