Provided is an illuminator including a light source unit having a light emitting surface, and a light guiding member guiding a light beam to an original disposed on an original plate. An intersection of a center axis of a light emitting surface and the original plate is separated from a reading position of the original in a sub scanning direction, the light guiding member includes an incident surface to which the light beam emitted from the light source unit is incident, a reflection surface that reflects the incident light beam, and an exiting surface exiting the incident light beam and the reflected light beam, and in the sub scanning section, the light beam having the highest light intensity in the light beams from the light source unit is deflected at the incident surface toward the reading position of the original with respect to the center axis.