There is provided a power output system 1 comprising an internal combustion engine 6, an electric motor 2 and a transmission 20 including two transmission shafts 11, 16 which are connected to the internal combustion engine 6. The electric motor 2 includes a stator 3, a primary rotor 4 and a secondary rotor 5. The primary rotor 4 is connected to either of the two transmission shafts 11, 16. The secondary rotor 5 is connected to drive shafts 9, 9. And, the other transmission shaft of the two transmission shafts 11, 16 transmits power to the drive shafts 9, 9 without involving the electric motor 2.