Abstract: A device as disclosed for automatically adjusting the brightness of a brake lamp which has a bulb mounted to the central portion of a reflector and a lens attached to the outer edges of the reflector. The device includes a hinge pin transversely mounted to the inside surface of the lens and a plate hinged to said pin, said plate having a curved shape corresponding to the inside surface of the lens and a plurality of holes, and adapted for normally closing the upper portion of the lens and selectively opening the lens with an inertia force acting on an automobile. Also, the device further comprises a weight mounted to the upper end of the plate for selectively opening the plate so as to increase the brightness of the brake lamp and an elastic member mounted to the hinge pin for normally biasing the plate so as to elastically return the plate to its original position.