An auxiliary power mechanism used for an electrically assisted bicycle is simplified and is easily attached to a bicycle body frame. An electrically assisted bicycle that can travel on pedaling forceassisted by electric power has a sprocket (2) for transmitting pedaling force to a drive wheel, an electric motor (37) for outputting electric power from its output shaft (37a), an assist gear (30) connected to the output shaft via a gear mechanism (40), and a pin (123) coaxially connecting the assist gear to the sprocket. The above construction components are arranged on a common base (50) and form a unit (11) that can be removably mounted on the body frame. A head portion of the pin is covered by an elastic member (129). The elastic member is provided in a region of engagement between at least either the sprocket or the assist gear and the pin. Electric power supplied to the assist gear is transmitted to the sprocket through the pin and the elastic member.