At least one beam emitted from a light source is deflected by a deflector, and the beam deflected by the deflector is imaged on a surface to be scanned by imaging optical system having a diffracting surface on at least one surface thereof. Of diffracted lights diffracted by the diffracting surface when the surface to be scanned is scanned by the beam, relative to diffracted light of an order used to form a spot on the surface to be scanned, the expanse of the stray light of one of unnecessary diffracted lights of the other orders undergoing surface-reflection by a refracting surface of the imaging optical system upon incidence on the surface to be scanned is designed to be wider in the sub-scanning direction than in the main scanning direction.