Abstract: This invention relates to a braking device for indoor exercise bicycles. In particular, this device is composed of a rotor, multiple rubber magnets, a wheel for the transmission belt, a stator, a coil, two pivot bearings, and an axle; when all of these components are assembled in place, the transmission belt can start the wheel for the transmission belt which drives the rotor in spin, producing relative motion between the rubber magnets of the rotor and the coil of the stator. There are magnetic flux loops between the rubber magnets and the stators. Hence, as the rotor spins, the coil crossed by the magnetic force lines will induce a potential, which by shorting the two ends of the coil starts a current and produces an inverse magnetic field, thus exerting a braking force upon the rotor or, in effect, achieving the end of consuming user energy.