To prevent leakage of light by pressing a plate member to a light source side by a prescribed pressure mechanism.

A cap 17 is pressed in the direction of an arrow G by a coil spring 20 arranged on the bottom of a frame 16, and the projection 12H of a light guide plate 12 is pressed on the fluorescent lamp side of the frame 16 as shown by the arrow G through the cap 17. Accordingly, even if the light guide plate 12 is extended or contracted, the light guide plate 12 is held so that the overlapping width of a prism sheet with a trimming or the overlapping width of the light guide plate 12 with the trimming is not changed. Shaking of the light guide plate 12 is also prevented. Since the pressure by the coil spring 20 can be surely transmitted to the projection 12H regardless of the size or form of the projection 12H, the projection 12H can be formed closer to the emitting surface side in a small size, so that the unnatural emitting phenomenon that the root of the projection 12H is observed from the emitting surface side can be reduced to a sufficient degree.