PURPOSE: To finely quantize a block with low definition and to visually preventing the block distortion and the quantization distortion.

CONSTITUTION: A blocking circuit 12 divides picture data into n×n blocks. A DCT circuit 13 forms the conversion coefficient by DCTing picture data. An activity detection circuit 51 detects the high definition of each block based on the ac component of the conversion coefficient. A weighting coefficient generation circuit 52 generates the weighting coefficient based on the high definition. A multiplier 53 multiplies the conversion coefficient by the weighting coefficient. A quantizer Qm quantizes the multiplied conversion coefficient by the quantization width with different quantization width, forming quantization data. An encoder CODm encodes each quantization data and forms encoding data. A buffer memory BUFm stores the encoding data for each sync block. A selector 54 selects the maximum encoding data without overflow at its output.