To provide a light backlight device having uniform luminance and chromaticity by using a solid-state three-color light-emitting element that emits red, green, and blue colors as a light source and improving light utilization efficiency, and to provide a display.

The backlight device comprises a first light reflecting plate that is divided into a main lightguide plate having a light reflecting surface on a light emission surface opposite to the light emitting surface and a first sub lightguide plate and a second one on both the sides via a first air layer at the center, while being installed on the rear of the main lightguide plate, and is installed on the front opposite to the main lightguide body of the sub lightguide plate for forming a light incidence surface on one end face that faces the first and second sub lightguide plates in the first air layer; a second light reflecting plate, installed on the rear that faces the first light reflecting plate in the sub lightguide body; a reflector for optically coupling the main lightguide plate and the sub lightguide plate, while being installed on both the end faces of the main and sub lightguide plates; and a light-emitting diode substrate, that is installed facing the rear of the sub lightguide plate and is installed by aligning light-emitting diodes for emitting a plurality of colors, at least in a row in the first air layer.