A light guiding plate guides light from a light source and irradiates the light onto a display portion. The light guiding plate has an incidence plane, an opposite end plane opposite to the incidence plane, an exit plane facing the display portion, and a reflection-exit plane opposite to the exit plane. A plurality of grooves are formed on the reflection-exit plane. The depth of the grooves monotonously increases from the incidence plane toward the opposite end plane. Each groove is defined by intersecting first and second planes. In each groove, the angle defined by the first plane and the exit plane is substantially equal to that of the other grooves. In each groove, the angle defined by the first plane and the second plane is substantially equal to that of the other grooves. This waveguide increases the brightness and improves the uniformity of brightness distribution.