An image forming apparatus includes an input part for receiving image data; an image processor for image-processing the input image data; an output part for outputting data processed at the image processor; and a controller for controlling the image processor to process the image using one of a plurality of image processing modes having a toner consumption lower than a toner density to use when the toner density to use for outputting the input image data exceeds a preset reference density. The plurality of the image processing modes includes a boundary image processing mode which outputs only boundaries of objects in the image data. Thus, the toner used for the image forming can be saved.