1. A fuzzy inference apparatus comprising: (a) a weighting means for evaluating the degree of matching of characteristic variables and having a plurality of rules formed with a first half and a second half using a membership function of a value between "0" and "1", said weighting means including means for evaluating a degree of matching of said first half from characteristic variables of a process inputted into said rules and for weighting said membership function of said second half according to the degree of matching; (b) a synthesizing means for synthesizing a new synthesized membership function at a present time t from (i) a previously synthesized membership function previously synthesized at a past time t-.DELTA.t with respect to said present time t and obtained from and fed back from a previous synthesizing operation carried out at said past time t-.DELTA.t and (ii) the membership function weighted by said weighting means; and (c) an inference value deciding means for deciding an inference value from said new synthesized membership function obtained by said synthesizing means.