To make an angle of attack of a fin appropriate in a reaction fin.

The reaction fin 7 is installed at the bow side from a propelling propeller at the stern side of a vessel. The reaction fin 7 includes first fins 7a-7c installed at the port side and second fins 7d-7f installed at the starboard side. When an angle made by the blade chord of each blade element to the blade chord of a root is denoted as a twist angle, the tip end of each of the first fins 7a-7c and the second fins 7d-7f has twist to the root so that the tip end has the twist angle.