An apparatus for fixing an optical film used in a back light to an outer frame of the back light. In the back light, a protrusion protrudes from a frame rim of the outer frame. A film extension extends from one side edge of the optical film and is placed on the surface of the frame rim near the protrusion. A clip is provided with a hole for inserting the protrusion to be mounted onto the frame rim such that the protrusion is inserted into the hole, thereby fixing the film extension of the optical film onto the frame rim.