A spacer is provided. The spacer includes an opening through which light passes and an internal surface that forms the opening, wherein the internal surface includes a first internal surface having a first curvature portion having a first predetermined curvature and a second internal surface disposed to face the first internal surface in a first direction, perpendicular to an optical axis direction, and having a second curvature portion having a second predetermined curvature, a distance between the first internal surface and the second internal surface is less than a distance between surfaces facing each other in the second direction, intersecting the first direction, and the first curvature portion and the second curvature portion are provided as a pair, corresponding to each other, to form an arc of a circle inscribed on the first internal surface and the second internal surface.