PURPOSE: To obtain a signal close to an ideal value as an output by setting a conversion equation from an input signal to an output signal as a linear combination of approximated equations taking characteristics of input and output.

CONSTITUTION: The relation between one input signal and a logic value is used as an approximated value (fn(Xn)), the characteristic of input and output is taken in an equation and the conversion equation is set as a linear combination of approximated equations so as to minimize the error of the converted value. The average color difference in the YMC space after the conversion from input color picture signals W, Ye, G, Cy into output color picture signals Y, M, C is decreased and the color after the conversion is seen similar to the color of an original. Thus, a difference between a theoretical value of an output signal yn (n is a positive integer) and the output signal yn obtained from the conversion even with respect to any input signal xn (n is a positive integer) is decreased and the physical quantity representing them is recognized to be of nearly equal degree.