To improve a convenience in assembling a rotor of an electric motor of a power transmission unit comprising the electric motor and a transmission mechanism.For this purpose, in the power transmission unit comprising an electric motor 2 having a rotor 13 arranged on an inner circumference of a stator 12 and concentrically with the stator 12 and a transmission mechanism 1 for transmitting power, a portion of a predetermined constructional element 7 of the transmission mechanism 1 protrudes toward the stator 12 or the rotor 13 side coaxially with the stator 12 or the rotor 13. The rotor 13 is loosely fitted onto an outer circumferential face of the protruding portion 7, and a clearance C is created between an outer circumferential face of the protruding portion 7 and an inner circumferential face of the rotor 13 into which a guide sleeve for guiding the rotor 13 in an axial direction is inserted.