To provide a liquid crystal display backlight and a liquid crystal display device, which can improve the heat dissipating effect, can reduce the thickness and weight of the device. The liquid crystal display backlight includes: an emission light guide plate for emitting illuminating light for a liquid crystal display panel from one face; one and other mixed-color light guides mounted on a same plane of other face of the emission light guide plate via a spacer; a heat-dissipating spatial area provided between the mixed-color light guide plates; light-emitting devices disposed within the heat-dissipating spatial area; and reflectors for guiding output light, propagated inside each mixed-color light guide plate, towards corresponding end faces of the emission light guide plate from outside end face of each mixed-color light guide plate. A holding plate for holding the light-emitting devices is provided, facing each mixed-color light guide plate with an airing gap provided therebetween.