The present invention is an optical scanning apparatus provided with a light source, an optical deflector that reflects and deflects a light beam from the light source, one or more lenses through which the light beam reflected by the optical deflector passes, a reflective mirror group which guides the light beam having passed through the lens, and an image bearing member on which the light beam having passed through the reflective mirror group forms an image, in which the reflective mirror group disposed in accordance with the light source is configured by including at least a third reflective mirror that reflects the light beam having passed through the lens toward the image bearing member, and the third reflective mirror and the image bearing member are arranged opposing each other across an optical path reflected by the optical deflector and passing through the lens.