ELECTRICALLY ASSISTED BICYCLE AND UNIT ADAPTED FOR USE IN ELECTRICALLY ASSISTED BICYCLE AND CAPABLE OF BEING ATTACHED TO BICYCLE BODY FRAME

An object of the present invention is to provide a simplified power assisting mechanism to be used in the power-assisted bicycle and facilitate the installation of the same mechanism to the bicycle body frame. A power-assisted bicycle which can travel with a pedaling force supplemented with an electromotive power, comprising: a sprocket 2 which is rotatable to transmit a pedaling force to a driving wheel; an electric motor 37 for providing an output of an electromotive power from an output shaft 37a thereof; an assist-gear 30 coupled to the output shaft 37a via a gear mechanism 40; and a pin 123 for coupling the assist-gear 30 coaxially with the sprocket 2. These components are arranged on a common base 50, forming a single unit 11 which can be attached to and detached from a bicycle body frame. A head portion of the pin 123 is covered with an elastic member 129. It is disposed in a region of the engagement between the sprocket 2 and the pin 123. Thus, the electromotive power supplied to the assist-gear 30 can be transmitted via the pin 123 and the elastic member 129 to the sprocket 2.