In a backlight of a liquid crystal display device, an elastic projecting part is provided in the inner side of an frame surrounding a perimeter of a light guide. The elastic projecting part functions to elastically fix the light guide on a supporting member, so that despite oscillation or external impact, the light guide does not move. Such an arrangement prevents breakage of an enclosure that is caused by external impact, or damage to an optical film that is provided on the upper surface of the light guide. Wrinkling of the optical film, which is caused by deformation of the light guide at high temperature, can be prevented. Also, the light guide does not separate from the supporting member during jarring movement.