A backlight unit 10 for an outside mirror of a vehicle may comprise: a light source unit 30 including at least one light source 31 disposed therein; a light processing unit 40 having at least one light passage 41 through which light emitted from the light source 31 passes while being diffused, and a reflective surface 42 for reflecting the light having passed through the light passage 41; and a photorefractive means 60 which is disposed at a light exit of the light processing unit and refracts light emitted from the light exit of the light processing unit to be oriented toward a driver's seat.