A backlight unit for blind spot warning lighting includes a printed circuit board (PCB) mounted with a light-emitting diode (LED), a bottom case having the PCB embedded on a lower portion thereof, and having an opened front surface, and a top case comprising a front plate covering a the front surface of the bottom case, and a guide protrusion formed to protrude backward from a rear surface of the front plate to guide the light from the LED to the front plate, and an object of the present disclosure is to provide a backlight unit, which can uniformly emit light while unifying the number of reflection and diffusion layers, and be advantageous for applying a hot wire, thereby improving visibility.