An electrically assisted bicycle which can be provided with a transmission mechanism in a so-called motor drive unit and can also minimize discomfort and shock to a rider during a gear change is provided. A speed reduction mechanism 25 provided in a motor drive unit 20 includes a low-speed reduction gear 36, a high-speed reduction gear 41, a low-speed one-way clutch 51 disposed between the low-speed reduction gear 36 and a resultant-force transmission member 29, a high-speed one-way clutch 52 disposed between the high-speed reduction gear 41 and the resultant-force transmission member 29, and a selection clutch 45 removably engaged with the high-speed one-way clutch 52. The selection clutch 45 is engaged with the high-speed one-way clutch 52 so as to place the high-speed one-way clutch 52 into a free state, whereas the selection clutch 45 is disengaged from the high-speed one-way clutch 52 so as to activate the high-speed one-way clutch 52.