To provide a new power transmission unit having excellent torque transmission efficiency, capable of low-speed/high-torque output to perform the starting/creep traveling or hill holding, and compactly formed while with a function of a power generator and a starter motor to have an excellent on- vehicle mountability.

This power transmission unit has an input circle pipe 32 connected to an input shaft 1, provided in an external diameter direction of an output shaft 2; a carrier connection circle pipe 34 connected to a carrier 12 and a sun gear connection circle pipe 36 connected to a sun gear 11 pinchingly provided inside and outside the input circle pipe 32; a rotor 16 of a power generating electric motor MG directly or indirectly installed to the sun gear connection circle pipe 36; a second clutch B-C/L provided between the output shaft 2 and the carrier connection circle pipe 34; a first clutch A-C/L provided between the carrier connection circle pipe 34 and the input circle pipe 32; and a third clutch C-C/L provided between the input circle pipe 32 and the sun gear connection circle pipe 36.