A side-lighting type surface light source device 100 includes a light source 40, a reflector member 50, a light guide plate member 30, and a fixing member 10. The reflector member 50 reflects a light ray from the light source, and includes opposed first end portion 520c and second end portion 520a, wherein the first end portion is arranged to project out of the second end portion. The fixing member 10 includes a fixing body 12 having a support surface (a back portion 12a) and a reflector holder 14 for holding the reflector member. The reflector member supports the reflective sheet 20 and the light guide plate member 30 with the first end portion thereof.