A power transmission unit for use in electric bicycles, the power transmission unit comprising: an input shaft caused to rotate by external force transmitted thereto; a transmission member provided on an outer periphery of the input shaft and coupled to, and configured to rotate along with, the input shaft; an output structure (31, 31C) configured to output rotational power of the transmission member; a one-way clutch; a rotational member coupled to either the transmission member or the input shaft and configured to rotate along with the input shaft; a rotator (632, 623B, 632C) configured to rotate while interlocking with the rotational member; and a detection unit (631, 631B, 631C) configured to detect a rotational state of the rotator (632, 623B, 632C), a rotational axis of the rotator (632, 623B, 632C) being located, in a direction perpendicular to an axis of the input shaft, at a different position from a rotational axis of the input shaft, the power transmission unit further comprising: a motor; and a speed reducer mechanism configured to transmit rotational power of a motor shaft of the motor to reduce a rotational velocity of the motor shaft, wh erei n the speed reducer mechanism includes a gear configured to rotate while interlocking with the motor shaft, characterised in that the one-way clutch is arranged between the transmission member and the output structure and configured to allow rotational power to be transmitted from the transmission member to the output structure only when the transmission member rotates in one direction with respect to the output structure; and a rotational axis of the rotator (632, 623B, 632C) is located, in the direction perpendicular to the axis of the input shaft, at a different position from a rotational axis of the gear. an input shaft caused to rotate by external force transmitted thereto; a transmission member provided on an outer periphery of the input shaft and coupled to, and configured to rotate along with, the input shaft; an output structure (31, 31C) configured to output rotational power of the transmission member; a one-way clutch; a rotational member coupled to either the transmission member or the input shaft and configured to rotate along with the input shaft; a rotator (632, 623B, 632C) configured to rotate while interlocking with the rotational member; and a detection unit (631, 631B, 631C) configured to detect a rotational state of the rotator (632, 623B, 632C), a rotational axis of the rotator (632, 623B, 632C) being located, in a direction perpendicular to an axis of the input shaft, at a different position from a rotational axis of the input shaft, the power transmission unit further comprising: a motor; and a speed reducer mechanism configured to transmit rotational power of a motor shaft of the motor to reduce a rotational velocity of the motor shaft, wh erei n the speed reducer mechanism includes a gear configured to rotate while interlocking with the motor shaft, characterised in that the one-way clutch is arranged between the transmission member and the output structure and configured to allow rotational power to be transmitted from the transmission member to the output structure only when the transmission member rotates in one direction with respect to the output structure; and a rotational axis of the rotator (632, 623B, 632C) is located, in the direction perpendicular to the axis of the input shaft, at a different position from a rotational axis of the gear. a motor; and a speed reducer mechanism configured to transmit rotational power of a motor shaft of the motor to reduce a rotational velocity of the motor shaft, wh erei n the speed reducer mechanism includes a gear configured to rotate while interlocking with the motor shaft, characterised in that the one-way clutch is arranged between the transmission member and the output structure and configured to allow rotational power to be transmitted from the transmission member to the output structure only when the transmission member rotates in one direction with respect to the output structure; and a rotational axis of the rotator (632, 623B, 632C) is located, in the direction perpendicular to the axis of the input shaft, at a different position from a rotational axis of the gear.