An image area of a main scanning line is segmented into a plurality of regions, with a first image being formed that has a first number of pixels in each of the plurality of regions, and a second image being formed that has a second number of pixels in each of the plurality of regions that differs from the first number of pixels. A scaling factor of an image in each of the plurality of regions is derived thereafter, in accordance with a difference in size between the first image and the second image in each region, and a differential between the first number of pixels and the second number of pixels. A determination is made with respect to a number of pixels to be either added to, or subtracted from, each of the plurality of regions, in accordance with a total number of correction pixels in order to adjust the size of the image of the main scanning line, and the scaling factor. The image data of each of the plurality of regions is corrected in accordance with the determined number of pixels in each region.