Provided is a back light unit. The back light unit includes a light guide plate having a light guide pattern part; at least one light emitting unit provided at a sidewall of the light guide plate, and irradiating light to the light guide plate; a keypad positioned over the light guide plate, and comprising a resin layer, a base resin layer, a character opening part having a character shape, a key assembly provided at one side, and a key adherence film provided on an upper surface of the base resin layer; and a printed circuit board comprising a metal dome switch positioned under a lower surface of the light guide plate.