A light pipe has an upper surface, a lower surface, side surfaces between the upper and lower surfaces, and light output means. At lease two of the side surfaces constitute incidence side surfaces each extending in left-right and front-rear directions of the light pipe. The light output means is formed in one of the upper and lower surfaces so that incident light to the one of the upper and lower surfaces from each of the incidence side surfaces is made to go out from the other one of the upper and lower surfaces by the light output means. The light output means comprises a repetitive structure of prismatic structures having ridgelines extending along each of the incidence side surfaces respectively. There is further provided a plane light source unit in which light sources are provided on the incidence side surfaces, respectively, of the light pipe, and there is still further provided a reflection type or transmission type liquid-crystal display device in which a liquid-crystal cell is disposed on the lower or upper surface side of the light pipe of the plane light source unit.