PURPOSE: To automatically assist the driving power with a motor by providing an auxiliary power motor, a pedal driving force detecting means, a means setting the upper limit value of the pedal driving force, and a control means controlling the pedal driving force within the preset value.

CONSTITUTION: The pedal position and the pedal driving force are read into a microcomputer 35 when an auxiliary mode selecting switch 23 is not turned off, and they are compared with the upper limit preset values for the subtraction process. The microcomputer 35 changes the duty ratio according to the subtracted value and feeds a signal to a motor driver 34. Switching is performed according to the signal in the motor driver 34, and a motor 11 is driven by the quantity corresponding to the pedal driving force exceeding the upper limit value. The power of the motor 11 is decelerated by a reduction gear 12 and rotates a pulley 14 via a one-way clutch 13, and the human power is assisted by a chain 10.