To provide an electromotive power assisted bicycle enabling an aerobic exercise with a light weight and simple mechanism, an electromotive power assisted bicycle 1 comprises a pedal effort detection means for detecting a pedal effort (99, 126), a drive unit 13, a force combining mechanism for transmitting a rotational torque of a speed reducing unit to a drive shaft, and a heart rate detection sensor 254. The drive unit 13 comprises an electric motor 37 and a speed reducing unit 35 having an electromagnetic clutch 250. In a mode enabling the aerobic exercise, if the detected pedal effort is higher than the pedal effort level enabling the aerobic exercise, the drive unit 13 drives the electric motor 37 under the condition of the electromagnetic clutch 250 being engaged and thus assists the pedal effort with the electric force. If the detected pedal effort is lower than said pedal effort level, the electric motor is turned off under the condition of the electromagnetic clutch 250 being engaged, and thus applies a loading force to the pedal effort by the rotational resistance of the motor. The pedal effort level can be adjusted in response to the detected heart rate of the rider.