An image processing apparatus which, when part of image data cannot be processed, is capable of extracting and processing substitute data having a predetermined relation with the image data that cannot be processed. The image processing apparatus processes an image data file including a plurality of image data portions. A data processing section of the apparatus sequentially processes the image data portions. When the image data portions include an image data portion that cannot be processed, another image data portion having attribute information associated with the image data portion that cannot be processed is extracted from the image data file. A CPU of the apparatus causes the data processing section to process the extracted image data portion as substitute image data.