In a liquid crystal display device which uses light emitting diodes as a light source, the present invention provides a light source which can be effectively housed in a housing casing of a backlight even when an area of a printed circuit board is increased by taking radiation of heat from the light emitting diodes into consideration. A printed circuit board is formed by arranging light emitting diodes on a flexible substrate. The printed circuit board is formed with an area larger than an area of a light incident surface of a light guide plate, and is arranged to face the light incident surface of the light guide plate in an opposed manner. The light guide plate and the printed circuit board are housed in the inside of a housing casing, the printed circuit board is made foldable by forming slits in the printed circuit board thus enabling housing of the printed circuit board in the housing casing in a folded shape. Due to housing of the printed circuit board in a folded shape, a contact area of the printed circuit board with the housing casing is increased thus enhancing a heat radiation effect.