PURPOSE: To reduce the torsional vibration of an engine by using two stage speed reduction gears.

CONSTITUTION: In the normal operational range S of a propeller shaft, the speed is reduced by using first stage gear in the range S1 from 1.0N to 0.6N. When the rotational speed is reduced under this state, a third order resonant point I/3 in the first node is generated in the vicinity of 0.4N. Therefore the third order torsional distortion in the first node is not generated in the rotational range S1 from 1.0N to 0.6N. The second stage gear is employed in the range S2 below 0.6N. At this time reduction ratio is 1/1.67 time that of first stage. The third order resonant point I/3 in the first node when the second gear is used is generated at the engine rotation of 0.4n, i.e. in the vicinity of 0.24N although the position is slightly moved up and down depending on the shaft system. In this method, no troublesome torsional vibration appears in the normal operational range from 1.0N to 0.33N.