A hierarchical knowledge base system is provided to enhance the efficiencies of constructing knowledge bases, making inferences, and understanding and analyzing the contents and state of a fault in a plant. Hierarchical knowledge bases store plant information on the states of plant equipment, media flowing through the plant, control instructions, and a plant system diagram. This information is used to make inferences for the detection of a fault in the plant, the identification of faulty equipment, and the cause of the fault, in stages from an upper layer to a lower layer, and the results of the inferences are displayed. The hierarchical knowledge base is constructed by synthesizing a knowledge base of an upper layer from a knowledge base of a lower layer.