To efficiently compress a composite image formed by composing a natural image and a non-natural image.

A line buffer 101 separates image data 131 read by an image scanner section 130 for reading image data into a plurality of regions. A determination section 120 determines the proportion of part occupied by regions having prescribed characteristics in the image 131. A quantization matrix selecting section 121 selects a quantization matrix used for compression of the regions on the basis of this proportion. An image data coder 103 compresses the regions by the selected quantization matrix. An attribute flag coder 102 compresses information which helps a decoder side to estimate that the region is coded by which quantization matrix.