In a driving unit for an electric motor driven bicycle, a motor (31) has an output shaft coaxial with a crank shaft (39) of a pedal. A planet roller reducer (40) has a sun roller shaft coincident with the output shaft. Through the planet roller reducer (40) and a two-stage reducing gear mechanism (50) for further reducing the output of the planet roller reducer (40), a torque of the motor (31) is transmitted to a driving chain sprocket (4). Provided that the torque of the motor (31) exceeds a predetermined pedaling torque level, the torque is transmitted to the driving chain sprocket (4).