1. An image forming apparatus comprising: an input part to receive image data; a storage part to store information of a preset reference density an image processor to generate a plurality of images by applying a plurality of image processing modes to the input image data respectively; a density calculator to calculate a toner density to apply to the plurality of images generated at the image processor, the density calculator being configured to calculate the toner density for all of the plurality of images only after all of the plurality of images have been generated; an output part to output an image; and a controller to select the image having the toner density lower than the reference density among the plurality of the images by comparing the toner densities calculated for the plurality of images generated at the image processor with the reference density, when the toner density to use to output the input image data exceeds the reference density, and to control the output part to output the selected the image.