A surface light source device comprising a linear light source (1), a light transmitting member (2) having an end surface of incidence located in the vicinity of said light source, a diffusing plate arranged on the front surface of said light transmitting member and a reflecting plate arranged on the rear surface of said light transmitting member; patterns formed as portions (6) having diffusing functions being arranged on the rear surface of said light transmitting member, and said portions having the diffusing function being sparse in area in the vicinity of the center of the end surface of incidence and becoming denser as the portions are farther from the end surface of incidence. The surface light source device is adapted so as to uniformalize luminance on the diffusing plate.