PURPOSE: To enable execution of measurement at low cost by obtaining an asymptotic value by transforming a nonlinear equation into a linear equation which expresses a tangent plane viewed from a space being higher by one dimension.

CONSTITUTION: A first means 2A of a computing device 2 calculates xin+10= Piobs-Pi(x10, x20,..., xn0)(i=1 to n) for an approximate solution xi0(i=1 to n) of a primary physical amount xi(i=1 to n). A second means 2B calculates a tangent plane Qi(x1, x2,..., xn, xn+1)=0(i=1 to n) at a point (x10, x20,..., xn0, xin+10)(i=1 to n) on a curved surface Piobs-Pi(x1, x2,..., xn)-xn+1=0(i=1 to n) in a space of an n+1 dimension. Next, a third means 2C calculates the solution of n-dimension linear simultaneous equations Qi(x1, x2,..., xn, O)=O(i=1 to n) and this solution is made to be a new approximate solution xi0(i=1 to n). Subsequently, a fourth means 2D makes processes by the first to third means be executed repeatedly until any of absolute values of xin+10(i=1 to n) turn to be set values or smaller.