The present invention provides an image forming apparatus and a controling method. This invention makes it possible to prevent any uneven density and degradation in resolution in reciprocal laser scanning using a movable mirror. The image forming apparatus forms an image on a photosensitive member by scanning the photosensitive member with a laser beam reflected by a movable mirror with reciprocating the movable mirror whose rotation angle is changeable in accordance with an electrical signal. The apparatus detects the rotation direction of the movable mirror, and weights, on the basis of therotation direction, image data of two lines along the sub-scanning direction in correspondence with the ratio of the scanning length after the start of laser write for a line of laser scanning to compensate the image to be formed using the movable mirror.