PURPOSE: To avoid signal deterioration by selecting a fine quantization characteristic of a smaller quantization step every time code quantity control processing is repeated so as to decrease the stored quantity of quantization distortion.

CONSTITUTION: Received picture data P are rearranged into picture data Pb in the unit of blocks by a block processing circuit 202 and transformed into transformation data Tj by a DCT device 203. A quantization decision circuit 207 outputs quantization characteristic designation data (r) whose code data quantity is almost an object quantity to control the quantization characteristic of a quantization device 204 and converts the data Tj into conversion data Dj for each block. Then the data are coded into data Cb subject to variable length coding and the data are outputted from a terminal 206, and the data (r) are outputted from a terminal 208. Furthermore, the data (r) received from an input terminal 209 are given to an inverse quantization device 212 and data Cb from an input terminal 210 are decoded by a decoder 211 and an identification number Dj for the same block as the input data to a coder 205. Then the data are subject to inverse quantization as data Tj and data P' are outputted.