PURPOSE: To generate always proper motor auxiliary force by measuring respectively a manpower in a cease period of motor auxiliary force and resultant force in an impression period, finding a target value of the auxiliary force to the manpower, and correcting the target value from a difference between it and an auxiliary force actual value when the auxiliary force is added.

CONSTITUTION: A crank pedal is installed on a crank shaft 32 of a motive power unit containing a permanent magnet type DC electric motor 22, and rotation of a driving shaft 30 is transmittied to a rear wheel 14 through a one-way clutch 40 and an interior trim type speed change mechanism 42. In such a bicycle having the electric motor, manpower driving force in a cease period of auxiliary force and resultant force of the manpower driving force and the auxiliary force in an impression period are detected by a manpower-resultant force detecting means 50, and a target value of the auxiliary force corresponding to the manpower driving force is stored in a memory 68A. By an auxiliary force operation correcting means 68B, an actual value of the auxiliary force obtained by subtracting the manpower driving force from detected resultant force is compared with the target value, and the target value is corrected, and the auxiliary force is controlled by an electric power circuit 68C in order to make it coincide with a corrected target value.