PURPOSE: To uniformize a quantization distortion of a screen by equalizing an average minuteness of a processing unit with approximately that of an entire screen.

CONSTITUTION: A memory 51 stores a picture data. An adder 53 adds a horizontal high frequency component of the picture data extracted by HPE 52h and 52v to a vertical one. A sorting circuit 55 sorts a block into four groups based upon the high frequency component. A memory 51 stores a block number for each group. A control circuit 57 divides the picture data stored in the memory 51 into blocks, where n×n pieces in spatial allocation are taken as one block, and controls so that a block ratio constituting a processing unit comes to be proportional to the number of blocks of each group when the picture data is read out of each processing unit consisting of the specified number of blocks. A DCT circuit 13 processes the picture data read out of each processing unit by a discrete cosine conversion to calculate a conversion coefficient. A quantization circuit 73 quantizes the conversion coefficient for outputting.