A backlight unit includes a light source having an emission region, a wiring board having the light source mounted thereon, a light guide plate having a side surface into which light from the light source enters, and a front surface from which the light exits, a light shielding adhesive tape adhering to the wiring board, and an optical sheet which overlaps with the front surface of the light guide plate. The front surface of the light guide plate includes an effective region serving as a planar light source and a light entering region ranging from the side surface to the effective region. The wiring board and the light-shielding adhesive tape each have a part positioned in the light entering region, and the optical sheet is arranged from the effective region to the light entering region. An end portion of the optical sheet overlaps with the light-shielding tape.