An image processing apparatus for decoding a compressed image data, the image data being divided to a plurality of tiles, each of which is a basic unit of process in encoding or decoding process, said apparatus comprising: a detector which detects an existence status of ROI within said compressed image data based on a frequency transform coefficient of said tile for every tile; a determiner which determines whether each tile is a ROI tile composed of only ROI, a non-ROI tile composed of only non-ROI, or a ROI boundary tile composed of ROI and non-ROI based on said existence status of ROI detected by said detector; a processor which shifts only frequency transform coefficient of said ROI within each tile to lower bit side for said ROI tile and said non-ROI tile determined by said determiner, and which shifts frequency transform coefficients of both ROI and non-ROI within each tile to lower bit side.