A power-assisted bicycle is provided to satisfactorily keep torque detection capability while reducing the pedal force of a pedal during pedaling if a battery has run out. A cylindrical human-power transmission member 28 having a magneto-striction generation portion 31b of a torque sensor 31 is disposed on the outer periphery of a crankshaft 7a. A combined force member 29 for combining a human driving force and an auxiliary driving force is disposed on the outer periphery of the crankshaft 7a. An auxiliary-driving-force transmission path from a motor 21 to the combined force member 29 has a deceleration mechanism 25 that includes a plurality of reduction gears 36c and 36d and reduction gear support shafts 36a and 36b. A one-way clutch 37 for interrupting a human driving force is disposed between the reduction gears 36c and 36d and the reduction gear support shafts 36a and 36b.
