A backlight device includes a light guide plate opposed to fluorescent tubes, a diffusion sheet stacked on the light guide plate, and lens sheets further stacked on the diffusion sheet. A tip end of a corner portion of the diffusion sheet has a shape defined by removing a portion of the sheet along a straight line spanning between two sides constituting the corner portion. In first of the lens sheets, a tip end portion of a corner portion formed by two sides has a shape defined by removing a portion of the sheet in a larger amount than in the diffusion sheet by a circular arc which projects outward. In a second of the lens sheets, a tip end of a corner portion formed by two sides has a shape defined by removing a larger amount than in the first of the lens sheets along a straight line.