An illuminated position adjusting method used in optical scanning apparatus, suppressing deterioration of imaging spot shape on scanning surface, and preventing undesirable light from arriving at the scanning surface, to form high-quality image. The optical scanning apparatus includes: a common deflector for deflecting beams emitted from light source units; and imaging optical units each including at least one imaging optical element and at least one reflector which are disposed in optical path of beam deflected by the common deflector, and image the beam on scanning surface. In an optical path in which the largest number of the reflectors are disposed among the optical paths guided to the scanning surfaces, the reflector which is disposed optically closest to the scanning surface is rotated in sub-scanning direction, to adjust the illuminated position of the beam, which illuminates the scanning surface, on the scanning surface in sub-scanning direction.