A backlight unit including a light guide plate, a linear light source disposed in the light incident side of the light guide plate, a reflective sheet disposed in the rear side of the light guide plate, and a heat spreader sheet disposed in the rear side of the reflective sheet. In this backlight unit, an edge of the heat spreader sheet is placed more inner than the light incident side surface of the light guide plate in the vicinity of an electrode portion of the linear light source. Furthermore another edge of the heat spreader sheet is substantially aligned with an corresponding side surface other than the incident side surface of the light guide plate.