A surface light source device includes a point source of light and a light guide plate causing light from the point source of light introduced from a light incident end face and emitted from a light emission surface to outside. The light guide plate is formed of a light introducing portion provided at an end of a light guide plate body. The point source of light is placed facing the light incident end face positioned at an end face of the light introducing portion. Along a region of the light introducing portion adjacent to the light guide plate body, a directivity converting portion is provided. The directivity converting portion is configured by arranging V-grooved directivity conversion patterns in an arc shape. A region of the light introducing portion has a thickness equal to that of the light guide plate body and has a surface positioned in the same plane.