Provided is a method of assembling and adjusting multi-beam scanning optical apparatus including a light source unit having multiple light emitting points, a deflecting unit configured to deflect multiple light beams emitted from the light source unit, and an imaging optical system configured to image the multiple light beams deflected by the deflecting unit on a surface to be scanned, the method including: adjusting incident positions of the multiple light beams in a sub-scanning direction, which are entered into an imaging optical element constituting the imaging optical system to adjust irradiated positions of the multiple light beams on the surface to be scanned; and adjusting the irradiated positions of the multiple light beams on the surface to be scanned in the sub-scanning direction without changing the incident positions.