1. A fuzzy rule acquisition method for a fuzzy inference system in which a fuzzy inference is achieved based on a fuzzy knowledge including fuzzy rules, each of the fuzzy rules described in a format having an IF part and a THEN part, and membership functions defining meanings of propositions described in the IF part and the THEN part, said fuzzy rule acquisition method comprising the steps of: (a) executing a fuzzy inference for at least one input value by using said fuzzy rules of said fuzzy knowledge, and obtaining a result of the execution of the fuzzy inference; (b) comparing the result of the execution of the fuzzy inference with a teaching value, and obtaining an inference error; (c) obtaining errors of said fuzzy rules by using said inference error by referring to at least one of combinations of IF parts of said fuzzy rules, combinations of THEN parts of said fuzzy rules, and both of the combinations of the IF part and the THEN part of said fuzzy rules; (d) determining whether a pair of said fuzzy rules are mutually contradictory, based on a pair of errors corresponding to said pair of fuzzy rules, by comparing the errors of each pair of fuzzy rules among all pairs of said fuzzy rules; (e) determining each pair of fuzzy rules as contradictory rules if a ratio between the errors of each pair of fuzzy rules is within a predetermined range and one of the errors is negative and the other of the errors is positive; (f) modifying a fuzzy rule determined as contradictory to dissolve the contradiction by comparing the areas corresponding to the IF parts of each pair of fuzzy rules determined as contradictory and modifying the IF part of the fuzzy rule determined as having a larger area by the comparison result.