1. A fuzzy inference processing apparatus comprising: a CPU for calculating m grades of if-part membership functions with respect to m input variables for each rule, the CPU outputs grade of 1 with respect to the input variable which is not defined in the rule; first synthesizing means, connected to said CPU, for synthesizing the m grades of the if-part membership functions of each rule to obtain a truth value of the if-part of each rule; calculators for storing then-part membership functions, for receiving the truth value of the if-part of each rule obtained by said first synthesizing means, and calculating a truth value of the then-part of the rule; and second synthesizing means, connected to said calculators, for synthesizing the truth values of the then-part of all the rules to obtain an inference result.