PURPOSE: To provide a new optical deflecting device which can effectively reduce the wind whistle of a rotary polygon mirror and exerts no substantial adverse influence upon optical characteristics of an image formation optical system.

CONSTITUTION: The side wall part of an enclosure which encloses the rotary polygon mirror 2 is formed of a material, whose refractive index is ≤1.5, into a transparent hollow cylinder 1, which is arranged having its center axis of curvature aligned with the center axis of rotation of the rotary polygon mirror 2 and meets a condition of 0<d≤0.0004.fMo, where (d) is the thickness and fM is the focal distance of the image formation optical system 5 of the optical scanning device in the direction corresponding to a main scan.