Image forming apparatus, which prevents the occurrence of displacement of the positions of pixels between image data items forming a synthetic image, and eliminates distortion of a character and color shift in character regions and non-character regions. An image size adjustment section obtains, when synthesizing a plurality of image data items to create one image, a remainder by dividing the number of vertical and horizontal pixels of input image data by a least common multiple of the ratio of resolutions of a plurality of image data items, for vertical and horizontal directions of the image. The input image data is processed by means of the result of computing the remainder, so that the remainder becomes 0 when obtaining the remainder by dividing the number of vertical and horizontal pixels of the input image data by the least common multiple of the ratio of the resolutions of the image data items.