1. An image forming apparatus that converts N-valued input image data representing an N-valued input image, and specifying pixel values of pixels of the N-valued input image, N being an integer equal to or greater than two, into output image data, the image forming apparatus comprising: a concentration determiner for taking each pixel as a pixel of interest, and determining a concentration for the pixel of interest by weighted addition of a pixel value of the pixel of interest and pixel values of the neighboring pixels arranged in an area centered on the pixel of interest; a ranking unit for assigning a rank to the pixel of interest and to each of the neighboring pixels responsive to the concentrations determined by the concentration determiner; and a pixel value distributor for redistributing pixel values among the pixel of interest and the neighboring pixels according to the ranks assigned by the ranking unit.