1. An approximate calculator for calculating an approximate value of a non-linear function using input data, comprising: decoder means for outputting m-bit data (m is a natural number) that represents a value corresponding to a slope of a coordinate straight line and outputting intercept data of said coordinate straight line based on said input data, said coordinate straight line interpolating said non-linear function for an interval that includes a value of said input data as one value of one of coordinates and having a slope of 2 n (n is an integer), said intercept data representing intercept of said coordinate straight line; shifter means for shifting said input data by &verbar;n&verbar; bits based on said m-bit data and outputting resultant data as first term data; and approximation output means for generating and outputting said approximate value of said non-linear function based on said first term data and said intercept data.