1. A fuzzy inference system comprising: a fuzzy inference unit including: a plurality of membership function generation circuits, each of an analog type operable in an electric voltage mode; a sweep signal generation circuit for generating a sweep signal; a plurality of switching means for switching inputs to the membership function generating circuits, respectively, between said sweep signal from said sweep signal generation circuit and an external input; and a plurality of membership function observing terminals for outputting outputs of said membership function generation circuits, respectively.