an encoder operable to encode image data and generate code data, for each small region, the code data being represented by a plurality of partial code data corresponding respectively to a plurality of layers, where partial code data at a higher layer is dominant for the quality of the corresponding small region;

a classification unit configured to divide the code data of each small region into a plurality of predetermined M(M>2) groups to classify the code data generated by said encoder, where each group includes partial code data of at least one layer and the plurality of groups are arranged in order of a group including a top layer to a group including a bottom layer;

a first determiner configured to, calculate a minimum value “i” satisfying ΣG(i)>R max, and determining an i-th group specified by the determined “i” as a boundary group, where G(i) is the total code amount of layers included in i-th groups in all of small regions of the image data to be encoded and Rmax is a target code amount of the image data to be encoded;

a selector configured to select all of the partial code data of layers included in first to (i−1)-th groups, excepting i-th to M-th groups, as basic code data for the image data to be encoded;

a second determiner configured to determine a boundary layer corresponding to a plurality of layers included in the i-th group in a small region of interest, and to select, among the partial code data of the plurality of layers included in the i-th group, partial code data of a higher layer than the determined boundary layer, as additional code data for the small region of interest; and

a code sequence forming unit configured to form a code sequence by combining the basic code data selected by said selector and each of the additional code data determined by said second determiner, as encoded data for the image data to be encoded.