Casing of optical scanning apparatus and optical scanning apparatus

A casing of an optical scanning apparatus housing a rotary polygon mirror having a plurality of reflecting surfaces and constructed to deflect a light beam emitted from a light source, the casing further housing an optical member including at least one selected from an imaging lens that causes the light beam deflected by the rotary polygon mirror to be imaged on a photosensitive member and a reflection mirror that guides the light beam deflected by the rotary polygon mirror to the photosensitive member, the casing having an opening that allows passage of the light beam for scanning the photosensitive member, the casing comprising:

• a mounting portion on which a sound insulation member is mountable, wherein the sound insulation member separates a first arrangement space of the optical member from a second arrangement space of the rotary polygon mirror, reduces propagation of sound generated by rotation of the rotary polygon mirror from the first arrangement space of the rotary polygon mirror to the second arrangement space of the optical member, and has a transparent window through which the light beam deflected by the rotary polygon mirror is emitted;
• a first support portion configured to support a transparent member which covers the opening of the casing in a case where the sound insulation member is mounted to the mounting portion, and which allows the light beam to pass from inside to outside the casing;
• a second support portion configured to support a transparent member which covers the opening of the casing in a case where the sound insulation member is not mounted to the mounting portion, which allows the light beam to pass from inside to outside the casing, and which is thicker than the transparent member to be supported by the first support portion for covering the opening of the casing in a case where the sound insulation member is mounted to the mounting portion;
• and
• a bearing surface of the second support portion that is to be brought into contact with the transparent member to be supported by the second support portion and that is located on an inner side of the casing, in which the optical member is arranged, is lower than a bearing surface of the first support portion to be brought into contact with the transparent member to be supported by the first support portion, so that a light beam emission surface of the transparent member to be supported by the first support portion and a light beam emission surface of the transparent member to be supported by the second support portion are on a substantially same plane.