This invention provides a technique of speeding up decoding processing by omitting several processes while reducing the influence of omission of processes on image quality in accordance with the variable magnification ratios at which images are to be decoded and displayed. For this purpose, an apparatus according to the invention analyzes the header of decoding target encoded image data. The apparatus then obtains information indicating how many times distortion suppression processing has been executed at the time of the generation of the encoded image data. The apparatus decides which process can be omitted, based on the set variable magnification ratio and the information extracted from the header, and generates an image on which an output is based, by executing the processes which are not to be omitted. The apparatus then re-sizes the image in accordance with the set magnification ratio.