To provide a surface light source device compatible with dimensional changes of a light guide plate.
In the surface light source device including a light guide plate having an incident surface as one of the side faces and a first side face as well as a second side face connected to the incident surface, and a chassis fixing the light guide plate, a plurality of either first notches or protrusions are fitted to the first side face of the light guide plate, and a plurality of either second notches or protrusions are fitted to the second side face. The chassis is provided with a plurality of first engagement parts engaged with the plurality of either first notches or protrusions, respectively, and a plurality of second engagement parts engaged with the plurality of either second notches or protrusions, respectively. A gap between the first engagement part and either the first notch or protrusion in a normal direction of the incident surface is the narrowest for the first engagement part nearest to the incident surface, and a gap between the second engagement part and either the second notch or protrusion in a normal direction of the incident surface is the narrowest for the second engagement part nearest to the incident surface.
Toshifumi Onuki
Akihiko Eguchi
Kazuhiko Naito