A power-assisted bicycle 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 a motor, the power-assisted bicycle being configured such that a cylindrical human-power transmission member is disposed on an outer periphery of a crankshaft that receives a human driving force from the pedal, the human-power transmission member receiving the transmitted human driving force and having a magneto-striction generation portion of a torque sensor for detecting the human driving force, a combined force member that combines the human driving force transmitted through the human-power transmission member and an auxiliary driving force from the motor is disposed on the outer periphery of the crankshaft, and a resultant force of the human driving force and the auxiliary driving force that are combined by the combined force member is transmitted to a rear wheel through a driving force output wheel coaxial with the crankshaft and an endless driving force transmission member looped over the driving force output wheel, the power-assisted bicycle comprising: a deceleration mechanism disposed on an auxiliary-driving-force transmission path from the motor to the combined force member, the deceleration mechanism having a plurality of reduction gears (36c, 36d) and a reduction gear support shaft (36a, 36b) that supports the reduction gears (36c, 36d), an interlocking cylinder that serves as a separate part of the human-power transmission member between the human-power transmission member and the combined force member, the interlocking cylinder rotating integrally with the human-power transmission member in engagement with the human-power transmission member, and a one-way clutch for interrupting an auxiliary driving force between the interlocking cylinder and the combined force member, the one-way clutch preventing transmission of an auxiliary driving force from the combined force member to the human-power transmission member and the crankshaft, characterized in that the power-assisted bicycle further comprises a one-way clutch disposed for interrupting a human driving force between one of the reduction gears (36c, 36d) and the reduction gear support shaft (36a, 36b) without transmitting a human driving force from the combined force member to the motor, and in that the interlocking cylinder and the human-power transmission member are fit to each other via one of a serration part and a spline part (28a, 30a). a deceleration mechanism disposed on an auxiliary-driving-force transmission path from the motor to the combined force member, the deceleration mechanism having a plurality of reduction gears (36c, 36d) and a reduction gear support shaft (36a, 36b) that supports the reduction gears (36c, 36d), an interlocking cylinder that serves as a separate part of the human-power transmission member between the human-power transmission member and the combined force member, the interlocking cylinder rotating integrally with the human-power transmission member in engagement with the human-power transmission member, and a one-way clutch for interrupting an auxiliary driving force between the interlocking cylinder and the combined force member, the one-way clutch preventing transmission of an auxiliary driving force from the combined force member to the human-power transmission member and the crankshaft, characterized in that the power-assisted bicycle further comprises a one-way clutch disposed for interrupting a human driving force between one of the reduction gears (36c, 36d) and the reduction gear support shaft (36a, 36b) without transmitting a human driving force from the combined force member to the motor, and in that the interlocking cylinder and the human-power transmission member are fit to each other via one of a serration part and a spline part (28a, 30a).