A power transmission unit is used for generating auxiliary driving force for an electric bicycle, and includes a motor, an input structure, a speed reducer, a case, a bearing, and a bearing support portion. The motor includes a motor shaft. The input structure includes an input shaft and configured to rotate along with the input shaft, the input shaft being caused to rotate by external force transmitted thereto. Thea speed reducer mechanism is configured to transmit a rotational power of the motor shaft while reducing a rotational speed. The case houses the motor, the input structure, and the speed reducer mechanism. The case includes a first divided part and a second divided part. The bearing is located inside the case. The bearing supporting portion, to which the bearing is attached, supports the speed reducer mechanism. The bearing supporting portion is attached to the first divided part.