In a backlight apparatus, laminated optical sheets are housed in a tray such that a wall portion thereof surrounds their circumferences so that the optical sheets receive light from fluorescent lights. Tips of one corner portion A of optical sheets preferably have a shape in which the tips are trimmed by line segments extending between two edges defining the corner portions. At the wall portion of the tray, an opposing surface is formed along the line segments of the corner portions so as to correspond to the shape of the corner portions having tips trimmed off. The opposing surface restrains the line segments.