Image data is stored into an image memory, then an image processing unit performs image processing on the image data stored in the image memory and generates information on a predicted amount of developer consumed in image formation, based on the image data resulted from the image processing. A printer forms an image by using the developer based on the processed image data, thereafter, the developer is supplied in correspondence with the information on the predicted consumption amount of the developer. Thus, primary image processing functions can be integrated on an image processing board, and the developer consumption amount can be predicted by the image processing board based on the image data.