An electrically assisted bicycle comprising a motor drive unit with a motor, the electrically assisted bicycle being capable of traveling with a combination of a human driving force generated by a pedal force from a pedal and an auxiliary driving force generated by the motor, wherein the electrically assisted bicycle comprises a crank shaft that receives a human driving force transmitted from the pedal and rotates about a different axis from the motor, the crank shaft has a human-power transmission member on an outer periphery of the crank shaft, the human-power transmission member receiving the transmitted human driving force, the crank shaft has a resultant-force transmission member on the outer periphery of the crank shaft, the resultant-force transmission member receiving a resultant force of a human driving force and an auxiliary driving force from the motor, the electrically assisted bicycle comprises a speed reduction mechanism the resultant force transmitted to the resultant-force transmission member through the speed reduction mechanism is transmitted to a rear wheel through a driving force output wheel coaxial with the crank shaft and an endless driving force transmission member looped over the driving force output wheel; characterized in that the speed reduction mechanism has pairs of reduction gears (27, 40; 38, 36; 39, 41)and a selection clutch; the speed reduction mechanism combining the human driving force and the auxiliary driving force and changing the gears for a resultant force of the human driving force and the auxiliary driving force, the motor drive unit contains the human-power transmission member, the resultant-force transmission member, and the speed reduction mechanism, the speed reduction mechanism includes: an intermediate shaft disposed in parallel with the crank shaft; a plurality of intermediate-shaft reduction gears (37, 38, 39) provided on the intermediate shaft; a low-speed reduction gear that is rotatable on the outer periphery of the crank shaft and receives a transmitted human driving force and a transmitted auxiliary driving force; a high-speed reduction gear that is rotatable on the outer periphery of the crank shaft and receives a transmitted human driving force and a transmitted auxiliary driving force; a low-speed one-way clutch disposed on a driving force transmission path from the low-speed reduction gear to the resultant-force transmission member; a high-speed one-way clutch disposed on a driving force transmission path from the high-speed reduction gear to the resultant-force transmission member; and the selection clutch removably engaged with the high-speed one-way clutch, and the selection clutch is engaged with the high-speed one-way clutch so as to place the high-speed one-way clutch into a free state, whereas the selection clutch is disengaged from the high-speed one-way clutch so as to activate the high-speed one-way clutch. a plurality of intermediate-shaft reduction gears (37, 38, 39) provided on the intermediate shaft; a low-speed reduction gear that is rotatable on the outer periphery of the crank shaft and receives a transmitted human driving force and a transmitted auxiliary driving force; a high-speed reduction gear that is rotatable on the outer periphery of the crank shaft and receives a transmitted human driving force and a transmitted auxiliary driving force; a low-speed one-way clutch disposed on a driving force transmission path from the low-speed reduction gear to the resultant-force transmission member; a high-speed one-way clutch disposed on a driving force transmission path from the high-speed reduction gear to the resultant-force transmission member; and the selection clutch removably engaged with the high-speed one-way clutch, and the selection clutch is engaged with the high-speed one-way clutch so as to place the high-speed one-way clutch into a free state, whereas the selection clutch is disengaged from the high-speed one-way clutch so as to activate the high-speed one-way clutch.