PURPOSE: To eliminate the redundancy of color data and to improve color reproducibility by non-linearly transforming specified chrominance signal data.

CONSTITUTION: An equation is used for converting R, G and B into new chrominance signal values L, C1 and C2. Here, the 1/3-th powers of the respective signal values are obtained for utilizing the fact that the visual sense characteristic of a human being responds in proportion to the 1/3-th power of a luminance signal as shown in the (L), (a) and (b) color specification systems of CIE (International Commission on Illumination). That is, the human being judges that a picture quantized to seven bits by L is nearly equal to the picture quantized to eight bits by G. The difference of the 1/3-th powers of R, G and B are taken as C1 and C2 for utilizing the fact that the visual sense characteristic of the human being deteriorates in color discriminating ability when a color saturation increases. Data after 1/3-th power conversion is quantized to seven bits and C1 and C2 after a subtraction are re-quantized to six bits as against the respective eight-bit inputs of R, G and B. Thus, a chrominance signal which is originally 24 bits per picture element is reduced to 19 bits.