To enable efficient encoding by encoding a bottom frequency sub-band for each frame and performing zero tree encoding while coupling the quantizing values of respective frames to one concerning high frequency sub-bands.

The respective frames of luminance Y and color differences Cr and Cb in a color still picture are decomposed to plural sub-bands by wavelet transformation. In this case, when an image format is 4:2:0, the number of times of division for the color differences Cr and Cb is legs than that of luminance Y for one and when the image format is 4:2:2 or 4:4:4, the numbers of times of division to all the luminance Y and color differences Cr and Cb are made equal. Then, the bottom frequency sub-band is quantized respectively independently to the luminance Y and the color differences Cr and Cb and adaptive predictive encoding is performed. Further concerning the high frequency sub-band, the amplitude of a coefficient is quantized, and three quantized values of the luminance Y and color differences Cr and Cb are coupled into one and encoded by a zero tree. Thus, efficient encoding can be attained.