To provide a ship propulsion performance improving device for giving propulsion performance improving effects even to a target ship which has a cross section shape showing a marked tendency to a V-shape and a wider deck.

The propulsion performance improving device includes fins 14, 16 and 15 protruded from positions immediately lateral to a bossing 3 obliquely upward to a hull center line C1 at an inclination angle of 15° or smaller, and provided on the starboard and port sides and on the hull center line, respectively, the three fins being twisted for changing a water flow in opposition to the rotating direction of a propeller when generating forward thrust. The resistance component of each fin itself falls below the propulsion performance improving component of each fin when operated to reduce a rotating flow behind the propeller, to improve the propulsion performance of a hull, thus bringing about the effects of reducing the required horsepower of a main engine required for operation of the ship.