IMAGE SIGNAL CODING DEVICE AND METHOD THEREFOR

Image signal coding device employing a quantization step size selecting unit which includes a multi-step quantization unit for performing quantization on the small blocks output from an orthogonal transform coding unit according to a plurality of predetermined quantization step sizes, multi-step run length coding unit for performing run length coding on the coefficients output from the multi-step quantizer, multi-step variable length coding unit for performing variable length coding on the coefficients output the multi-step run length coding unit, and a multi-step accumulating unit for accumulating the code length output from the variable length coding unit in units of small blocks, and for accumulating predetermined reference small blocks in units of large blocks. A normalizing unit performs normalization by using the code length accumulated in small block units and the code length accumulated in the reference large block unit, and allocates a new target code length to each small block. The quantization step size selecting unit selects a quantization step size capable of outputting the closest value to the target code length of the normalizing unit among a plurality of quantization step sizes given respectively to the small blocks output from the multi-step accumulating unit.