1. A method of determining fuzzy inference rules for an adjusting device to adjust output characteristics of a membership value determiner in an inference device to be used in an outer system, said inference device comprising a system variable input device for receiving input signals from said outer system and for generating an input vector from said input signals to the inference device; the membership value determiner for generating a membership value indicative of the degree of attribution of the input vector to an IF part of a fuzzy inference rule of an IF-THEN type, the input vector being received from the system variable input device; an individual inference quantity determiner for generating a first inference quantity of the input vector corresponding to the THEN part of a fuzzy inference rule, the input vector being received from the system variable input device; a final inference quantity determiner for determining a final inference quantity by processing values attained from the inference quantities generated by the individual inference quantity determiner and the corresponding membership values; and an adjusting device, receiving said input vector and said observed output from said system variable input device and said final inference quantity, for adjusting output characteristics of the membership value determiner, wherein said membership value determiner includes a signal processing network having input terminals receiving input vectors from the system variable input device, output terminals and an artificial neural network structure including a plurality of multiple input-single output signal processors each receiving plural respective outputs from one or more others of said signal processors and providing one output to one or more others of said signal processors; each of the multiple input-single output signal processors having an artificial neural network structure comprising a signal processor which includes a memory for storing a plurality of weighting coefficients to define membership values; a plurality of multipliers for weighting the input vector from the system variable input device with the weighting coefficients read from the memory; at least one adder for generating output data by adding plural weighted input vectors from said multipliers; and a threshold processor for generating an output as a membership value by clipping the output data from said adders within a predetermined range; the method comprising the steps of: (a) receiving and clustering the input vectors from the system variable input device and providing class numbers based on degree of similarity of the input vector, said class numbers corresponding to the output terminals of the membership value determiner respectively; (b) calculating an ideal output vector on the assumption that the ideal output vector is output from only the corresponding output terminals of the signal processing network corresponding to the class numbers provided as the result of clustering; (c) obtaining a difference between the ideal output vector and said first inference quantity from the signal processing network when said network receives the input vector from the system variable input device; (d) altering the weighting coefficients stored in the memory in order to decrease the obtained difference; (e) reiterating the above steps until the difference becomes less than a predetermined value so that a proper output characteristic is obtained for the signal processing network; and (f) fixing the output characteristic of the membership value determiner based on the output characteristics of the signal processing network after the reiterating step (e) is performed.