A power transmission unit includes an electric motor including a rotor arranged on an inner circumference of a stator and concentrically with the stator and a transmission mechanism for transmitting power. A portion of a predetermined constructional element of the transmission mechanism protrudes toward the stator or the rotor side coaxially with the stator or the rotor. The rotor is loosely fitted onto an outer circumferential face of the protruding portion, and a clearance is created between an outer circumferential face of the protruding portion and an inner circumferential face of the rotor into which a guide sleeve for guiding the rotor in an axial direction is inserted. The structure improves a convenience in assembling the rotor of the electric motor of the power transmission unit including the electric motor and transmission mechanism.