The invention provides a surface light source device, the surface light source device comprises: a point light source (13); a light guide plate (1) in which at least one end in the lengthwise direction of the plate is an incident plane (2), the two ends in the thickness direction are set as an emission plane (7) and a back plane (8), respectively, and the light emitted from the point light source(13) is introduced from the incident plane (2) and spreads over nearly the entire area of the emission plane (7) and is emitted; and a reflector (14) for further reflecting the light, from the light radiating from the point light source (13), which has been reflected by the incident plane (2) in order to irradiate the incident plane (2). The incident plane (2) is provided with an elliptical arc (10) which is a concave surface in the form of an elliptical arc and has the same shape as the elliptical arc which forms part of a mother ellipse (31) having the point light source (13) and the reflector (14) as the two focal points. The point light source (13) is arranged on one line and the reflector (14) on the other line of the two ellipse focal point lines (11, 12) on which the two focal points of the mother ellipse (31) move in the width direction of the light guide plate (1).