Assembling performance of a motor contained in a case contiguously to a transmission mechanism in a power transmission unit is enhanced. The power transmission unit comprises a motor (2) having a rotor (13) disposed on the inner circumferential side of a stator (12) concentrically to the stator (12), and a transmission mechanism (1) for transmitting power wherein a part (7) of a predetermined constitutional member in the transmission mechanism (1) projects to the side of the stator (12) or the rotor (13), and the rotor (13) is attached concentrically to the stator (12) while being guided by the projecting portion. The rotor (13) can be attached under a state substantially supported at the opposite ends even if the space of a chamber for containing the motor (2) has one closed end.