A surface light source device has a light source, and a light guide plate introducing light of the light source from a light incidence surface and emitting the light from a light exit surface to outside. The light source being is provided at a position facing the light incidence surface of the light guide plate. The light guide plate includes a light introducing part for enclosing the light from the light source entering from the light incidence surface and a light guide plate main body having a thickness smaller than a maximum thickness of the light introducing part, provided so as to be continued to the light introducing part, and emitting the enclosed light from the light exit surface by light emitting portion to outside.