An illumination apparatus for use in an image reading apparatus that forms a reduction image of a read area of an original on an image pickup element by an imaging optical system, includes: a light source illuminating the original; and a light guide guiding light to the original; wherein the light guide has a first surface on which light is incident or that diffuses light, a second surface from which light exits toward the read area, and a third and fourth surfaces arranged between the first and second surfaces in a sub-scanning section perpendicular to a longitudinal direction of the light guide, the third surface arranged on a side closer to an optical axis of the imaging optical system, and the fourth surface arranged on a side farther from the optical axis, wherein the third and fourth surfaces are reflective surfaces having the same paraxial power which is appropriately set.