The power transmission unit and installation procedures for this

The power transmission unit with an electric motor having a rotor in an inner circumference of a stator and is arranged concentrically with the stator, and a transmission mechanism for transmitting power, characterized in that: projecting a portion of a predetermined structure element of the transmission mechanism to the stator or rotor side and the projecting portion acts as a guide portion for concentrically inserting the rotor into the stator.