To provide a backlight device capable of preventing creases and flexure from being generated on an optical sheet even when the optical sheet is extended or constricted by a change of ambient temperature.

There are provided a light source 16 that emits light, an optical sheet 14 that adjusts the optical path of the light emitted from the light source 16 in such a way that a liquid crystal panel 12 is irradiated with the light from behind, and a frame 13 for attaching the optical sheet 14 on the back side of the liquid crystal panel 12. The optical sheet 14 is supported at one point on the frame 13. When the optical sheet 14 is generally rectangular, advisably it is supported on the frame 13 substantially at the center of one side of the optical sheet 14. For stable fitting, preferably the optical sheet 14 is supported on the frame 13 substantially at the center of that side of the optical sheet 14 which, in actual use of the device, is located at the top with respect to the direction of gravity.