An image forming apparatus includes a resolution conversion unit that integrates n pixels that are contiguous in the sub-scanning direction, and that determines an average value of pixel values of the n pixels as a pixel value of integrated pixels, a quantization unit that quantizes the pixel value of the integrated pixels to N levels, an image analysis unit that performs first determination processing for determining whether the difference between the pixel values of the n input pixels exceeds a threshold value set in advance, and second determination processing for determining whether a direction in which the pixel values of the n input pixels becomes greater is a forward direction of the sub-scanning direction or an opposite direction, and a pixel selection unit that determines n output pixels, based on the results, from a quantization result of the integrated pixels.