To provide a power plant which is capable of improving the driving efficiency and electric power-generating efficiency thereof. A first transmission 20 is connected between the output shaft 3a of an internal combustion engine 3 and driven parts DW and DW, which are connected to each other. A generator-motor 30 includes a stator 32 for generating magnetic fields and first and second rotors 31 and 33, and carries out energy input and output between the stator 32 and the first and second rotors 31 and 33 during generation of rotating magnetic fields. Along with the energy input and output, the rotating magnetic fields and the first and second rotors 31 and 33 rotate while maintaining such a linear speed relationship that the difference between the rotational speed of the magnetic fields and that of the second rotor 33, and the difference between the rotational speed of the second rotor 33 and that of the first rotor 31 are equal to each other. One of the first and second rotors 31 and 33 is connected between the output shaft 3a of the engine 3 and the first transmission 20, and the other to the driven parts DW and DW.