A backlight device includes a main light guide plate, auxiliary light guide portions provided on the side of an opposed surface of the main light guide plate, and light sources arranged to emit light into the auxiliary light guide portions. A reflector is arranged to surround the introduction surface of the main light guide plate and the auxiliary light guide portions. Furthermore, an optical path forming member is provided on the inside of the reflector so as to form a dedicated optical path from the auxiliary light guide plate to the main light guide plate. Thereby, the light emitted from one auxiliary light guide portion is prevented from entering into another auxiliary light guide portion.