An electric power-assist system for a manually operated vehicle, such as a bicycle, has a pedal with Hall sensors for producing a signal representing a rotational speed of the pedal, and an electric motor for power assisting the driving force of the bicycle. A speed counter, speed decision block, up/down counter, and comparator are provided so that the power assist by the electric motor increases or degreases, as the speed of the manual controlling device increases or decreases, respectively.