An image forming apparatus is operable in a first mode in which a photosensitive drum is rotated at a first rotational speed and a second mode in which the photosensitive drum is rotated at a second rotational speed slower than the first rotational speed. In this image forming apparatus, the light-emitting portions are controlled such that the number of light-emitting rows for multiply exposing regions, each of which corresponds to one pixel on the photosensitive drum in the first mode, is greater than that in the second mode.