According to one embodiment, a backlight unit includes a light source, a first lens, a second lens, and a light guide unit. The light source emits light which is diffused as the light travels in a first-A direction, the light being diffused in a second direction intersecting the first-A direction, and a third direction intersecting the first-A direction and the second direction. The first lens and the second lens are arranged in an optical path of the light. The first and second lenses control the light such that the first lens controls a beam width of the light in the second direction, and the second lens controls a beam width of the light in the third direction, thereby converting the light into parallel light.