Patent ID: 8092066

Claim:
A surface light source device that combines a light emitting element and a light guide plate to form a light emitting element/light guide plate unit, and that combines a plurality of light emitting element/light guide plate units to emit planar light of a rectangular shape, wherein: the light guide plate comprises: a light emitting area that has a square shape from a plan view and that comprises a surface of an emission surface and a back surface provided on an opposite side of the surface; an extended area that is positioned extending outward from a pair of neighboring sides in four sides defining an outer rim of the light emitting area; and an incidence surface area that allows light from the light emitting element to enter an end part lateral surface of the extended area; the extended area and the incidence surface area have symmetrical shapes with respect to one of diagonal lines of the light emitting area and an extending line of the one diagonal line; one of the plurality of light emitting element/light guide plate units is combined with another light emitting element/light guide plate unit by: assuming that the light guide plate of the one light emitting element/light guide plate unit is a first light guide plate and the light guide plate of the other light emitting element/light guide plate unit is a second light guide plate; making orientations of the first light guide plate and the second light guide plate uniform; aligning a side, which is one of the four sides defining an outer rim of the light emitting area of the second light guide plate and in which the extended area is formed, with a side which is one of the four sides defining an outer rim of the light emitting area of the first light guide plate and in which the extended area is not formed; placing the extended area of the second light guide plate in a back surface side of the light emitting area of the first light guide plate; and positioning the side in which the extended area of the first light guide plate is not formed, on the second light guide plate; and a reflective sheet is arranged on a substantially entire area of at least the back surface side of the light emitting area of the first light guide plate, and, in a position where the first light guide plate and the second light guide plate overlap, is arranged between a back surface side of the first light guide plate and a surface side of the second light guide plate.