Abstract: A driving power transmission system has a rotary base supported for rotation as one with a driving shaft rotated in forward and reverse directions at controlled rotational velocities and a driving power transmitting member rotatably supported by the driving shaft. The rotary base and driving power transmitting member are interconnected by an elastic connecting member. A rotational position control member is provided for controlling the rotational position of the driving power transmitting member with respect to the rotary base. During constant speed rotation, the driving power transmission system transmits the rotational driving power of the driving shaft through the elastic connecting member to the driving power transmitting member to prevent torque ripples produced in the driving shaft from being transmitted to a driven unit.