The backlight unit whose luminance life can be extended without increasing the cost and the power consumption includes: a light source unit to which a light source is mounted; a light guide plate which converts light from the light source to a surface light beam and emits the beam; and supporting members provided between the light source unit and the light guide plate, wherein each of the supporting members is a member having a function of shortening the distance between the light source unit and the light guide plate by the changes generated over time.