A light pipe includes rectangular upper and lower surfaces, an incidence side surface which is one of side surfaces between the upper and lower surfaces, and light output means formed in one of the upper and lower surfaces so that light incident on the incidence side surface exits from the other one of the upper and lower surfaces through the light output means. A radius of curvature of a corner portion formed at a point of intersection between the incidence side surface and each of side surfaces adjacent to the incidence side surface is not larger than 0.3 mm. A plane light source unit in which a linear light source is disposed on the incidence side surface of the light pipe is provided. A transmission type or reflection type, or reflection-transmission double type liquid-crystal display device which has at least such a plane light source unit and a liquid-crystal cell is provided.