In a liquid crystal display device having the so-called side-edge type backlight system which comprises: a liquid crystal display panel; a light guide plate being disposed opposite to a main surface of the liquid crystal panel; and a linear lamp being disposed along one of sides of the light guide plate, the present invention provides a plurality of grooves at a corner area of an edge of the light guide plate along which the linear lamp is disposed so that the plurality of grooves are extended in a direction slanted to the edge of the light guide plate. In accordance with this structural feature, the present invention enables a high quality image display performance of the liquid crystal display panel by suppressing the display irregularity appearing therein effectively.