An electromotive power assisted bicycle includes a drive shaft rotated by a pedal effort a primary sprocket fixed to the drive shaft for transmitting the pedal effort to a drive wheel; a secondary sprocket fixed to the drive shaft coaxially with the primary sprocket; a pedal effort detection sensor; an electromotive power output unit box detachably attached to the body for outputting electromotive power in response to the detected pedal effort; a power sprocket coupled to a rotating output shaft of the electromotive power output unit box; an auxiliary chain stretched between the secondary sprocket and the power sprocket; a supporting section for rotatably supporting the drive shaft; and a unit mounting bracket on which the electromotive power output unit box is mounted, the unit mounting bracket being fixed to the supporting section with the drive shaft penetrating through the unit mounting bracket.