The present invention is directed to a backlight device using light emitting units (7) in which a large number of light emitting diodes (12) are mounted, which comprises a reflection plate (16) for reflecting rays of light which have been emitted from a light emitting unit (11), wherein the reflection plate (16) is composed of a bottom surface reflection plate (16a) for reflecting rays of light emitted from the light emitting units (7) from the bottom surface direction, side surface reflection plates (16b) to (16e) for reflecting rays of light emitted from the light emitting units (7) from respective forward and backward, and left and right side surface directions, and corner reflection plates (16f) to (16i) in which the bottom surface reflection plate (16a) and the side surface reflection plates (16b) to (16e) are connected to cover the respective corner portions, and serves to permit rays of light incident onto a display panel (1) to be uniform to realize uniformity of brightness of the entire surface thereof.