An image forming apparatus comprising: an image-data acquiring unit configured to acquire image data composed of a plurality of pixels; a resolution converting unit (350a, 350b, 350c, 350d, 350e) configured to convert a resolution of the acquired image data into a higher resolution than the resolution of the image data; a position determining unit configured to perform an additional address value determining process of determining an additional address value of an additional pixel to be added on the basis of main-scanning directional addresses of pixels composing the converted image data and sub-scanning directional addresses calculated based on the main-scanning directional addresses; a density determining unit (350a, 350b, 350c, 350d, 350e) configured to determine a predetermined density as the density of the additional pixel on the basis of a density of a pixel located at a position corresponding to the determined additional address value in the acquired image data and a shift amount of the pixel shifted to the sub-scanning direction, wherein said shift amount is determined on the basis of the sub-scanning directional address; a correcting unit configured to perform a correction process of adding the additional pixel having the determined density to the determined position; and a scaling unit configured to control the position determining unit and the correcting unit so as to scale the image data up or down by causing the position determining unit and the correcting unit to repeatedly perform the additional address value determining process and the correction process with respect to each of sub-scanning lines of pixels and then repeatedly perform the additional address value determining process and the correction process with respect to each of main-scanning lines of pixels. an image-data acquiring unit configured to acquire image data composed of a plurality of pixels; a resolution converting unit (350a, 350b, 350c, 350d, 350e) configured to convert a resolution of the acquired image data into a higher resolution than the resolution of the image data; a position determining unit configured to perform an additional address value determining process of determining an additional address value of an additional pixel to be added on the basis of main-scanning directional addresses of pixels composing the converted image data and sub-scanning directional addresses calculated based on the main-scanning directional addresses; a density determining unit (350a, 350b, 350c, 350d, 350e) configured to determine a predetermined density as the density of the additional pixel on the basis of a density of a pixel located at a position corresponding to the determined additional address value in the acquired image data and a shift amount of the pixel shifted to the sub-scanning direction, wherein said shift amount is determined on the basis of the sub-scanning directional address; a correcting unit configured to perform a correction process of adding the additional pixel having the determined density to the determined position; and a scaling unit configured to control the position determining unit and the correcting unit so as to scale the image data up or down by causing the position determining unit and the correcting unit to repeatedly perform the additional address value determining process and the correction process with respect to each of sub-scanning lines of pixels and then repeatedly perform the additional address value determining process and the correction process with respect to each of main-scanning lines of pixels.