Provided is an illumination apparatus that applies light to an original, the illumination apparatus including: a light source; and a rod-shaped light guide including: an incident surface that receives the light from the light source; a reflection surface provided with a diffusion portion that diffuses the light incident on the incident surface; and an emission surface that emits, to the original, the light diffused by the diffusion portion. An area of the light guide is reduced by an inclination of the light guide from the incident surface side to a middle part in a longitudinal direction of the light guide, and the light guide is provided with the diffusion portion from the incident surface side to the middle part on the reflection surface.