A fin that generates thrust in the forward motion direction by utilizing rising, falling, and swirling flows near a rudder surface is attached to a rudder to improve the propulsion performance. The invention provides a finned rudder, disposed aftward of a screw propeller that rotates clockwise as viewed from the stern side during forward motion, for changing the course of a ship and provided with a first fin and a second fin in respective rudder surfaces. A first end of the first fin is attached at a position higher than a center position of the screw propeller on a leading edge side of a central portion of the rudder surface, and a first end of the second fin is attached at a position lower than the center position of the screw propeller on the leading edge side of the central portion of the rudder surface. A second end of the first fin extends at an upward inclination to a position inside the rotation radius of the screw propeller where the upward flow is strong, and a second end of the second fin extends horizontally to a position inside the rotation radius of the screw propeller where the downward flow is strong.