Power unit, saddle-riding type vehicle and method for controlling a power unit

Provided are: engine 6; electric-powered generator 66 with a regeneration function and power running function; a stepped transmission mechanism that shifts transmission of rotational power output from engine 6 and electric-powered generator 66; output shaft 73 that outputs the rotational power as a driving force; first clutch 74 that transmits the rotational power or disconnects the transmission of the rotational power from at least one of engine 6 and electric-powered generator 66 to output shaft 73; and battery 67 that supplies power to electric-powered generator 66 to drive electric-powered generator 66 and that can accumulate power generated by electric-powered generator 66. During the transmission by the transmission mechanism, electric-powered generator 66 is driven to add torque to engine 6 in a direction accelerating synchronization with the rotation of output shaft 73.