A first mark for indicating the direction of the rotational unbalance of a propeller shaft is provided on the propeller shaft while a second mark for indicating the direction of the rotational unbalance of a companion flange is provided on the companion flange, which is to be connected to the propeller shaft at a rotation-transmitting device. One of the directions of the rotational unbalances indicated by the first mark and the second mark is set as a direction along which the rotating mass becomes larger while the other is set as a direction along which the rotating mass becomes smaller, and one of the first mark and the second mark is provided over an angular region with a predetermined angle around an axis.