To improve the estimation accuracy of a fuzzy inference model by automatically generating a polygonal membership function consisting of a quadrilateral or more.

In a step 1, a membership function (e.g. N=5) of an N-polygon is supposed and serial numbers Pi are allocated to constitutional points occupying respective corners in the order of input values. In a step 2, the coordinate values of constitutional points P1, P5 on both ends are obtained by following equations (X1, Y1)=(Xmin, 0) and (X5, Y5) = (Xmax, 0). In a step 3, the coordinate value of a constitutional point P3 arranged on the center between both the points P1, P5 is obtained by following equations X3=X1+α3x (X5-X1) and Y3=1 (provided that α3x is a parameter of 0 to 1). In a step 4, the coordinate values of remaining points P2, P4 are obtained by following equations X2=X1+α2x (X3-X1), Y2=Y1+α2y (Y3-Y1), X4=X3+α4x (X5-X3), and Y4=Y3+α4y (Y5-Y3) (provided that α2x or the like is a parameter of 0 to 1).