PURPOSE: To improve the heuristic knowledge expression capability by using the storage level of a conclusion part and a minimum grade value of a condition part as another condition part to perform multistage inference.

CONSTITUTION: A maximum value selecting part 209 takes minimum grades h1 and h2 of respective rules as the input to obtain a minimum value h0 and updates the value in a storage means 210. At the time of next inference, the storage means 210 outputs the stored grade h0 to a minimum value selecting part 2. A function device 10 is the membership function of the conclusion part, and the top is deleted with the output h1 of a minimum value selecting part 1 by a low value preferring device 110 to obtain a synthesized ambiguous set of rules (conclusion). A low value preferring device 120 deletes the top of the function device 10 of the membership function of the conclusion part by the output h2 of a minimum value selecting part 2 to obtain a synthesized ambiguous set of rules (conclusion). High values are selected from two synthesized ambiguous sets obtained in this manner by a high value preferring device 100 to obtain an output ambiguous set. In this case, this is the membership function of fuzzy self-holding output.