An emitting light device of the present invention is realized on the bicycle to generate and conserve electric power, to light the emitting light device. In practice a driven wheel combined with rotating portion (i.e. rear wheel) of the bicycle to form a center-pivot type of axis-joint rotating structure. A bicycle dynamo includes 1. an actuated wheel and a dynamo with a set of differential gears between them, 2. a PCB, and 3. a battery accumulator. A transmission belt is rotated around the driven and actuated wheels; an emitting light base has conducting cords connected to PCB of the dynamo. As the driven wheel rotates, the actuated wheel is driven to rotate by the transmission belt. The speed ratio of transmission is changed by a set of differential gears to run the dynamo generate DC electricity and conserve electricity for battery. DC electricity can be applied to light LED.