A backlight unit is capable of preventing optical sheets from experiencing deformation due to thermal expansion and the like. The backlight unit includes a backlight chassis arranged to house a lamp via a diffusion plate and optical sheets, where a thickness of the optical sheets is less than a thickness of the diffusion plate, the backlight chassis including a support surface arranged to support the diffusion plate and the optical sheets, and a frame arranged to hold the diffusion plate and the optical sheets with the support surface, the diffusion plate and the optical sheets being interposed between the frame and the support surface, and the backlight chassis has a positioning piece arranged to position and support the diffusion plate at a predetermined position with respect to the support surface and not to position and support the optical sheets.