A treadmill includes a drive roller disposed in a tread base, a treadmill belt engaged with the drive roller, and a wheel attached to the drive roller and rotated in concert with the drive roller, and an electric generating device is attached to the tread base and includes a fan device attached to a spindle, and the spindle is engaged with the wheel for allowing the spindle of the electric generating device to be driven and rotated by the wheel and the drive roller and the treadmill belt in order to generate the electric energy, and the fan device is also to be driven and rotated by the wheel and the drive roller and the treadmill belt without the other electric energy, in order to generate the circulating or ventilating air to cool the user.