To prevent rod projections from turning backward into grooves where they come from, at points where the angle of the grooves cut in a rotor is changed greatly.

The projections are projected axially near recessed points of the comma-shaped grooves in the side faces of the rotor, projections whose projecting extents are smaller than those of the rod projections are provided separately therefrom projecting axially near the rod projections in their opposite side, or in theirs both sides. When the rod projections come to the furthest positions or the closest positions from the output shaft of the comma-shaped groove, projections of a planetary gear which is approximately in contact with the projections in the rotor side in such a state as turning into from the rotating direction side; and when a force turning the rods projection backward to the grooves, where they come from act thereon, the projections of the rotor abut against the projections of the planetary gear to stop their traveling farther, so as to prevent opposite entering.