1. A fuzzy feedback control system for regulating a process value of an object with improved stability and accuracy, said control system comprising: means for generating a deviation between an actual process value from the object and a set value; means for generating a variation of the deviation; means for determining a fuzzy control rule including: means for representing a reference straight line in a phase plane with said deviation and said variation of said deviation as coordinate axes, a speed type control output variation of proportional/integral control being zero, and said representing means dividing said phase plane into rectangles equal in size in such a manner that vertices of said rectangles are on said reference straight line and a line parallel with said reference straight line; and means for regulating a variation of control output in correspondence to a distance between said reference straight line and said parallel line which is so defined as to be proportional to absolute values of said deviation and said variation of said deviation; means for regulating antecedent membership functions concerning said deviation and said variation of said deviation as fuzzy variables, with centers of isosceles triangles of said antecedent membership functions being deviation components and deviation variation components of coordinates of centers of said rectangles; means for regulating consequent membership functions concerning a control output variation as fuzzy variables, with centers of isosceles triangles of said consequent membership functions being defined by a distance between said reference straight line and said parallel straight line; means for determining, by fuzzy inference, a variation of said control output using said fuzzy control rule, said antecedent membership functions, said consequent membership functions, a representation of said deviation, and a representation of said variation of said deviation; and means for applying the control output, as varied by fuzzy inference, to control the object.