1. An adaptation degree operation device in which a plurality of membership functions are set within defined sections of an input data range, said sections having boundaries located at peaks of said membership functions, said plurality of membership functions being set by stored data in a memory representing said peaks and, for each section, by stored data in said memory defining the gradient of a pair adjacent and overlapping portions of membership functions associated with said section, said pair of overlapping portions of said membership functions within a section being symmetrical with respect to a center of the section, said device comprising: a judging unit for determining a section to which applied input data belongs; an operation unit for finding in said determined section a first degree of adaptation of said applied input data with respect to a first membership function of the section using peak and gradient data stored in said memory for said determined section and associated with said first membership function, and finding a second degree of adaptation of a second membership function of the determined section based on the found first degree of adaptation; and a weighting operation unit for applying weighting data for said corresponding first and second membership function to said found first and second degrees of adaptation.