A LCD comprises a surface light source device of side light type for auxiliary lighting. A fluorescent lamp is turned on to supply primary light to be introduced into a guide plate. A back face (upper face) of the guide plate provides a light control face including an emission promoting surface to promote emission from an illumination output face (lower face). The illumination output face provides a specular face without scattering power. Illumination light L2 introduced into the guide plate escapes gradually from the back face and the illumination output face on the way of propagation.