the entrance surface of the first focal lens is transmitted through the other first focal lens symmetrically disposed relative to the first focal lens which reflected the beam of laser light with respect to the polygon mirror 1 and is projected onto the photosensitive drum which is not to be exposed by this beam of laser light as in the case of the above described optical scanning apparatus in accordance with the prior art in this embodiment therefore this unwanted portion of the beam of laser light which hereinafter will be referred to as flare is blocked by providing the housing 5 which supports the first focal lens 2 a with a rib 6 having a slit 6 a shown in fig1 in this case the rib 6 constitutes a flare blocking member more specifically the housing 5 is provided with the rib 6 and one of the surfaces of the rib 6 is used as the referential surface positioning means with which the first focal lens is placed in contact to accurately position the first focal lens therefore it is unnecessary to provide the optical scanning apparatus with a dedicated flare blocking component moreover with the interposition of no component it is possible to improve the optical scanning apparatus in vertical measurement accuracy making it easier to block the flare in the drawing the height h 1 or the slit 6 a is set to be the.