PURPOSE: To provide the orthogonal transmission coding system of a picture in which production of block distortion at a flat part of a reproduced picture is suppressed and the quality of the reproduced picture at high compression is improved.

CONSTITUTION: An adaptive device 33 uses a DCT coefficient obtained from a DCT transformation device 32 to discriminate a macro block belonging to a flat parted of a picture, picture element data in the macro block are sub- sampled with respect to the macro block belonging to the flat part and subject to DC transformation and the DCT coefficient is outputted to a quantizer 34 and the DCT coefficient in a sub block in the macro block is outputted to the quantizer with respect to the macro block not belonging to the flat part. The quantizer 34 selects a quantization table depending on whether or not the macro block belongs to the flat part of the picture to implement quantization. Through the constitution above, the resolution is reduced for the flat part of the picture to implement fine quantization and rough quantization is implemented for the part other than the flat part while keeping the resolution.