Image forming apparatus and method for reducing the difference of image qualities between the image data read by a plurality of reading units

An image processing controlling unit calculates an image processing parameter to reduce an image quality difference between image data read by a first reading unit and a second reading unit depending on a reading characteristic of a reading unit indicated by attached information (reading-unit information) received together with the calculating request and a writing characteristic of a printer. The calculated image processing parameter is stored in a memory. Next, when printing the image data, a middleware unit causes a printer to print the image data based on the image processing parameter held in the memory.