A power unit which makes it possible to reduce power transmitted from a prime mover to a driven part via an electrical path, to thereby increase the efficiency of driving the driven part. A first rotating machine of the power unit inputs and outputs energy between a stator and first and second rotors thereof, via magnetic circuits formed by generation of a rotating magnetic field, and the rotating magnetic field, and the rotors rotate while maintaining a linear relation in which respective differences in rotational speed between the rotating magnetic field and the second rotor, and between the second and first rotors are equal. The rotors are mechanically connected to a prime mover and a transmission, respectively. A second rotating machine of the power unit is mechanically connected to a drive part without via the transmission, and electrically connected to the stator.