A power-assisted bicycle that is adapted to travel 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 comprising: a force combining member on an outer periphery of a crank shaft that receives a human driving force transmitted from the pedal, the force combining member combining the human driving force and the auxiliary driving force generated by the motor; the human driving force and the auxiliary driving force being combined by the force combining member into a resultant force transmitted to a rear wheel through a driving force output wheel coaxial with the crank shaft and an endless driving force transmission member wound around the human driving force output wheel; a deceleration mechanism having a plurality of gear portions (21c, 29b, 36c, 36d) that are engaged with each other on a transmission path of an auxiliary driving force from the motor to the force combining member; a motor drive unit that includes a unit casing containing the motor, a central portion of the crank shaft, the force combining member, and the deceleration mechanism; a control board contained in the unit casing of the motor drive unit, the motor drive unit containing an oil guard cover that serves as a partition between a region of the deceleration mechanism and prevents oil on the gear portions of the deceleration mechanism from penetrating into the region of the control board; and a cylindrical human-power transmission member disposed on the outer periphery of the crank shaft so as to receive the transmitted human driving force, the force combining member receiving the human driving force transmitted via the human-power transmission member, characterized by the human-power transmission member having a magneto-striction generation portion for a torque sensor for detecting the human driving force, the cover having a wall portion as a partition between a region of a large-diameter gear portion provided on the force combining member and a region of the torque sensor. the power-assisted bicycle comprising: a force combining member on an outer periphery of a crank shaft that receives a human driving force transmitted from the pedal, the force combining member combining the human driving force and the auxiliary driving force generated by the motor; the human driving force and the auxiliary driving force being combined by the force combining member into a resultant force transmitted to a rear wheel through a driving force output wheel coaxial with the crank shaft and an endless driving force transmission member wound around the human driving force output wheel; a deceleration mechanism having a plurality of gear portions (21c, 29b, 36c, 36d) that are engaged with each other on a transmission path of an auxiliary driving force from the motor to the force combining member; a motor drive unit that includes a unit casing containing the motor, a central portion of the crank shaft, the force combining member, and the deceleration mechanism; a control board contained in the unit casing of the motor drive unit, the motor drive unit containing an oil guard cover that serves as a partition between a region of the deceleration mechanism and prevents oil on the gear portions of the deceleration mechanism from penetrating into the region of the control board; and a cylindrical human-power transmission member disposed on the outer periphery of the crank shaft so as to receive the transmitted human driving force, a force combining member on an outer periphery of a crank shaft that receives a human driving force transmitted from the pedal, the force combining member combining the human driving force and the auxiliary driving force generated by the motor; the human driving force and the auxiliary driving force being combined by the force combining member into a resultant force transmitted to a rear wheel through a driving force output wheel coaxial with the crank shaft and an endless driving force transmission member wound around the human driving force output wheel; a deceleration mechanism having a plurality of gear portions (21c, 29b, 36c, 36d) that are engaged with each other on a transmission path of an auxiliary driving force from the motor to the force combining member; a motor drive unit that includes a unit casing containing the motor, a central portion of the crank shaft, the force combining member, and the deceleration mechanism; a control board contained in the unit casing of the motor drive unit, the motor drive unit containing an oil guard cover that serves as a partition between a region of the deceleration mechanism and prevents oil on the gear portions of the deceleration mechanism from penetrating into the region of the control board; and a cylindrical human-power transmission member disposed on the outer periphery of the crank shaft so as to receive the transmitted human driving force, the force combining member receiving the human driving force transmitted via the human-power transmission member, characterized by the human-power transmission member having a magneto-striction generation portion for a torque sensor for detecting the human driving force, the cover having a wall portion as a partition between a region of a large-diameter gear portion provided on the force combining member and a region of the torque sensor.