To increase the life of a chain, and to eliminate the need for providing a shaft center of a driving gear more than necessary.

This bicycle is provided with a chain mechanism X composed of a front side gear 30 connected to a pedal 61; a read side gear connected to a center shaft 73 of a rear wheel; a chain 32 engaged with the front side gear 30 and the rear side gear, and transferring pedal depressing force input from the pedal 51 to the rear wheel; and a driving gear 33 receiving driving force from a motor and engaged with the chain 32 to transfer the driving force to the chain 32. The driving gear 33 is engaged with a lower chain 32b in such a way as to lift up a part of the lower chain 32b of the chain 32, and a tensioner 34 pressing generally downward the chain 32 between the lifted up part of the lower chain 32c and a part engaged with the rear side gear of the chain 32 is engaged with the chain 32.