ILLUMINATION UNIT, ORIGINAL READING DEVICE, AND IMAGE FORMING APPARATUS

PROBLEM TO BE SOLVED: To provide a compact illumination unit that is thinner than a conventional one without deteriorating light usage efficiency.SOLUTION: An illumination unit 10 comprises: a light source 1 arranged in a shape of an array; a light guide plate 2 having an incident end face on which light emitted from the light source 1 is made incident, a side face for guiding incident light made incident from the incident end face, and an exiting end face 23 in non-parallel with the incident end face; and a mirror 3 to emit illumination light on a surface to be illuminated on a contact glass 6 in a shape of a line. The illumination unit is configured so as to illuminate a belt-like area 5 to be illuminated by light transmitted from a portion of a side face 22 of the light guide plate 2 and light transmitted from the exiting end face 23 and reflected at the mirror 3, so that an illumination unit which has high efficiency and is low in height can be achieved.