Abstract: A mathematical relation between base variables (x1, x2, . . . , xp), when a set of input data d is composed of p base variables (x1, x2, . . . , xp), and a plurality of data sets d (i) of such data set d is inputted, the data sets d(i) are distinguished by an input data distinguishing parameter i. When victors in a q dimensional space, mapped from the input data through q base functions (f1, . . . , fq), form a plane, a mathematical relation can be a linear combination of the base functions. A set of base functions (f1, . . . , fq) are prepared. Sets of the values F(i)=(F1i, . . . , Fqi) of the base functions corresponding to the input data d (i) are acquired. The sets F(i) are considered points in a q dimensional space. Direction cosine of a mapping plane is acquired through cofactors of a determinant of these points, or by solving an eigenvalue problem for determining a plane, the square sum of the perpendicular lines from these points to the plane is minimum.