An LCD backlight has a light source that emits a laser light, a lens, a mirror, a rotary polygon mirror, a scanning lens and a return mirror constituting a transforming optical system that transforms the laser light from the light source to a linear light. A light guiding plate allows the linear light to enter and emits two-dimensional light in the horizontal direction. An emission end face of the light source is disposed in a rear face side of the light guiding plate so that a surface normal line thereof is in the horizontal direction or is inclined downward from the horizontal direction. The lens, mirror and scanning lens are disposed in the rear face side of the light guiding plate, so that surface normal lines of the light transmission surface and light reflection surface thereof are all in the horizontal direction or are inclined downward from the horizontal direction.