PURPOSE: To reduce the size and the weight of a counter-rotating device suitable for propelling a marine vessel, by having two propellers coaxially arranged, rotated in the reverse directions to one another by specifying the torque burden of front and back propellers.

CONSTITUTION: An engine output shaft 4a of a main engine 4 and a rear propeller 3 of a marine vessel are directly connected together by an inner propeller shaft 5 in a counter-rotating propeller device, while a front propeller 2 and the engine output shaft 4a are connected together through a counter-rotating mechanism 6, and the front and rear propellers 2, 3 are rotated in the reverse directions to one another. An external propeller shaft 7 of hollow cylindrical form is linked with the engine output shaft 4a through counter-rotating gears 8a-8d, and an elastic joint 9, in the counter-rotating mechanism 6. The blade area as well as the pitch angle of each propeller blade 2a, 3a are changed so that the ratio of burden torques T1, T2 becomes 1:1. The burden torque of the front propeller 2 is thus reduced, while the size of the counter-rotating mechanism 6 is reduced.