Provided is a multi-beam optical scanning apparatus, which is capable of reducing an inter-light-beams illuminance difference at the same image height on a surface to be scanned and forming a high-definition and high-quality image where no unevenness in density of image exists, and an image forming apparatus using the multi-beam optical scanning apparatus.In the multi-beam optical scanning apparatus that brings a plurality of light beams emitted from the plurality of light emission portions into incident on the deflecting surface of the light deflector so as to exceed a width in a main scanning direction of the deflecting surface, when performing the emission light amount control for each of the plurality of light emission portions so that illuminances of the plurality of light beams become same at every image height on the surface to be scanned, the control means changes emission light amounts of the plurality of light emission portions at a maximum scanning angle, thereby reducing the inter-light-beams illuminance difference on the surface to be scanned.