This invention provides a technique to turn off a distortion suppression function within the range in which image quality degradation is allowed, in order to increase the processing speed at the time of decoding. For this purpose, an apparatus of the invention rewrites parameter information in a header with information indicating that no distortion suppression processing has been performed, and decodes encoded image data including the rewritten header, when the header contains parameter information indicating that the distortion suppression processing has been performed only before the first frequency conversion.