An illumination device includes a light source composed of a light emitting element
and an intermediate light guide, and a light guide plate that receives light from
the light source through a light incident face and that emits the light propagating
therein from an emergent surface. A reflecting surface of the light guide plate
has a plurality of prism grooves formed in stripes in plan view, each prism groove
having a gently inclined face and a sharply inclined face inclined more sharply
than the gently inclined face. A 2-coefficient, which is the
product of the number of the sharply inclined faces per unit length of the light
guide plate, and the length of the sharply inclined faces, is increased away from
the light source in the light guide plate.
