To provide an image forming apparatus capable of properly correcting a main scanning scale factor without degrading a required printing quality.

In a main scanning scale factor correcting process, for each of one or more correction points (at l-th, m-th, and n-th pixels) on each of lines along which scanning is carried out on a photosensitive drum 11 by laser light, the final bit data of pixel-division-modulated pixel data of a pixel immediately preceding each correction point is added as the leading bit data of the pixel-division-modulated pixel data of a pixel located at the correction point. The same processing as above is sequentially performed on pixel data of pixels located subsequently to the correction point to sequentially shift the pixel-division-modulated pixel data of pixels to pixel data of the respective following pixels by bit, to thereby generate pixel data of a new pixel to be added on each of lines. The generated pixel data of the new pixel is outputted in synchronism with an image clock of a fixed frequency.