PURPOSE: To reduce block distortion by obtaining a difference between orthogonal transformation data and quantization data to obtain additional information and correcting inverse transformation data of a block having block distortion based on the additional information in the case of decoding.

CONSTITUTION: Original image data are received continuously by using, e.g. horizontal 8 picture elements × vertical 8 picture elements as a unit. The data are divided in frequency components in an orthogonal transformation circuit 12 and coded and compressed at a high efficiency in a quantization circuit 14 and a coding circuit 16. An additional information generating circuit 24 calculates a difference between the conversion data outputted from the orthogonal transformation circuit and quantization data outputted from the quantization circuit 14 to detect a difference from a data compression rate for each block of original image data, that is, the occurrence of discontinuity between blocks, i.e., block distortion in advance and the result is outputted as additional information. A correction circuit 26 corrects inverse transformation data outputted from an inverse orthogonal transformation circuit 22 based on the additional information to provide decoding image data with less block distortion.