PURPOSE: To prevent the generation of visual, distortion by using a small quantization width multiplier in a block where an edge is present in a relatively flat part.

CONSTITUTION: An activity deciding part 13 detects the edge in a flat part by comparing the activities of the present block with the adjacent block and decides the activity of the present block. A bit distribution part 14 distributes bit numbers to the respective blocks corresponding to values for indicating the complication of the blocks such as the dispersion and the dynamic range of picture element values inside the blocks. A pattern selection part 15 selects a pattern for minimizing overload distortion at the time of quantization from the patterns, for allocating bits to conversion coefficients, prepared in plural numbers corresponding to the distributed number of bits. A multiplier selection part 16 selects a multiplier for minimizing quantization distortion from the plural quantization width multipliers. In this case, a matrix for which an initial quantization width matrix is multiplied by the quantization width multiplier and the bit allocation pattern selected at the selection part 15 are used for the quantization.