A backlight unit includes a lamp housing, a plurality of elongate lamps received therein, and a cooling member having a heat-absorbing part and a heat-radiating part. The heat-absorbing part includes a plurality elongate heat-absorbing portions having a light reflecting function and arranged alternately with the elongate lamps. The heat-radiating part extends from the elongate heat-absorbing portions of the heat-absorbing part and is disposed outside the lamp housing. The cooling member has a heat-radiation function as well as a luminescence assistance function.