A multi-beam light scanning apparatus includes incident optical systems each of which allows a light beam to enter a deflection surface of a rotational polygon mirror in a sub scanning section from above and below directions obliquely with respect to a surface perpendicular to a rotation axis of the rotational polygon mirror at a finite angle, and a light source unit which is disposed for each of the incident optical systems and has light emitting portions. A light emitting portion corresponding to printing of a head line in the sub scanning direction among the light emitting portions of the light source unit that are disposed obliquely in the upward direction, and a light emitting portion corresponding to the printing of the head line in the sub scanning direction, that are disposed obliquely in the downward direction are different from each other in the sub scanning direction.