An image forming apparatus having a reduction layout function has as its object to suppress any image degradation in recompressed image data. To achieve this object, an image processing method of this invention has the following arrangement. More specifically, an image processing method of generating reduced layout image data by using a plurality of image data as configured image data includes a step (step S1008) of decoding the plurality of configured image data which are compressed by using individual compression parameters, a step (step S1007) of extracting, on the basis of the compression parameters of the plurality of configured image data, a common compression parameter to compress the plurality of configured image data and assigning the common compression parameter, a step (step S1009) of recompressing the plurality of configured image data by using the assigned common compression parameter, and a step of compositing the plurality of recompressed configured image data as image data in a single page (step S1010) and outputting the image data (step S1012).