PURPOSE: To encode a picture data with a high efficiency while keeping picture quality by deciding a quantization step with a different method from brightness information and chrominance information.

CONSTITUTION: An inputted picture data is subject to RGB-YCrCb conversion by a chrominance signal conversion circuit 2 and the result is stored in a line memory 4. In this case, the color differnece signal CrCb is subsampled by a subsampling circuit 3 in a preset ratio and the result is stored. A coefficient after 2-dimension discrete cosine transformation by a 2-dimension dicrete cosine transformation circuit 5 is subject to linear quantization by a quantization circuit 6. A quantization step size in this case stored in a quantization table changeover circuit 7. Then Y the data quantization table for a photograph or Y the data quantization table for a character is selected depending on the result of discrimination of character/photograph depending on a brightness signal Y. Moreover, the quantization table for CrCb data is selected with respect to the color difference signal CrCb.