A reverse rotation preventing device comprising: a fixed ring disposed in proximity to the external peripheral surface of a coil spring; an input coupling rotatably driven; an output coupling, to which is fixed an output shaft; and a reinforcing ring on the input side and a reinforcing ring on the output side, which cover the opposite ends of the coil spring, respectively. Engaging pawls are projected and bent in the axial direction at the both ends of the coil spring, respectively, and an engaging recession for engaging the engaging pawl is formed on each of both reinforcing rings. An engageable portion of the input coupling is engaged with an engageable pawl formed on the reinforcing ring on the input side to decrease the diameter of the coil spring, so that the output shaft is rotated. An engageable portion of the output coupling is engaged with an engageable pawl formed on the reinforcing ring on the output side to increase the diameter of the coil spring, so that the output shaft is prevented from rotating reversely.