To allow a finer quantization control according to the property of an image within a macroblock, quantization parameter values are allowed to be changed in units of sub-blocks equal to or smaller than the macroblock in a similar manner as in motion compensation and orthogonal transform processes. A finer-tuned quantization control is performed, for example, by selecting fine and coarse quantization parameters respectively for corresponding sub-blocks if a plurality of images having different properties coexist within the macroblock.