A backlight unit that makes it sure to assemble many optical members in a casing in a right order with their right attitude in its assembly stage is provided. Each of the optical members to be assembled in a predetermined order has a protruding part including at least one protrusion. A count of the protrusions of each member is increased according to the predetermined assembly order. The casing has a depressed part for receiving the protrusions of the members, the depressed part including notches or depressions corresponding to the protrusions of the optical members. Each of the notches or depressions has a depth in a stacking direction that varies according to stacking state of the optical members. The notches or depressions may be formed to penetrate through the casing to thereby confirm easily the order and attitude of the assembled members after the assembly operation.