1. A light scanning apparatus comprising: a light source which emits a light beam; a deflector which has a deflection mirror oscillating in sine motions about a drive axis approximately orthogonal to a main scanning direction, the deflection mirror reflecting the light beam emitted from the light source so as to scan the light beam reciprocally in a main scanning direction; and an imaging optical system which exhibits an arc-sign theta lens characteristic, and focuses the light beam deflected by the deflector on a surface to be scanned so as to form a beam spot on the surface, the surface being driven in a sub scanning direction approximately orthogonal to the main scanning direction and including an effective scan region spanning across a predetermined width in the main scanning direction, wherein a diameter of the beam spot in the sub scanning direction is equal to or larger than the maximum scanning pitch in the sub scanning direction within the effective scan region.