A starter generator has an electric rotating machine having a rotating shaft and designed to carry two reverse operations—motor and generator. A first clutch is connected between the rotating machine and the first rotating shaft for transmitting rotations of the rotating machine to the first rotating shaft. A second clutch is connected between the rotating machine and the second rotating shaft for transmitting rotations of the second rotating shaft to the rotating machine. A continuously variable transmission is provided for changing the number of rotations transmitted from the second rotating shaft to the rotating machine. The rotating machine is supplied with electric power to rotate the rotating shaft, or the number of rotations of the rotating shaft is kept constant by controlling a transmission ratio of the continuously variable transmission.