PURPOSE: To reduce the manufacturing cost, to improve the design, and to improve the safety by devising the constitution of a motor driving system relative to an manual driving system to dispense with a sprocket and a chain for the motor driving system.

CONSTITUTION: In a bicycle with the auxiliary driving force to which the auxiliary driving force by a motor, a motor driving system relative to a manual driving system integratedly comprises a battery, a motor 18, and a reduction gear 19. The whole structure of the motor 18, an output rotary shaft 18a, the whole structure of the reduction gear 19, and a sun gear 9a which is an output rotary shaft are provided with a hollow part 34. A crank shall 10a of a pedal 10 is held in the hollow part 34 rotatably through bearings 11-16, and rotated by the sun gear 9a through a unidirectional clutch 12. A stepping force detecting sensor 24 is connected to a rotary ring 24 to detect the tension of an input driving system chain 33 according to the torque of the stepped pedal 10.