To provide an optical scanner which enables improvement of an illuminance correcting accuracy at the time of performing correction to make constant an image surface illuminance distribution on a photosensitive drum, and to provide an image forming apparatus.

The image forming apparatus is equipped with the photosensitive drum 19, an optical scanning part 2, and an image surface illuminance measuring part 61. The optical scanning part 2 calculates a quartic approximate expression having measured values of illuminances on a plurality of image heights of the photosensitive drum 19 subjected to quartic approximation to a plurality of light amount set values at a laser. Next, an illuminance difference between the illuminance on an arbitrary image height of the photosensitive drum 19 and the illuminance on at least one or more other image heights is calculated for the plurality of light amount set values by using the quartic approximate expression. Then, an illuminance correction value is calculated on the basis of the illuminance difference, and a light amount of laser to be ejected from the semiconductor laser 12 is controlled on the basis of the illuminance correction value.