A backlight unit includes a planar light source member that has a light-emitting surface and that emits light having directivity in a perpendicular direction to the light-emitting surface. An optical member has a light-receiving surface opposed to the light-emitting surface of the planar light source member and a light-emitting surface opposite to the light-receiving surface. The light-receiving surface has a multiplicity of prisms. By the prism action, the optical member emits the light having directivity in the perpendicular direction from the planar light source member as light having directivity in at least two directions on opposite sides of the perpendicular direction. The directions of light directivity are changed by varying the apex angle of the prisms.