PURPOSE: To increase the steering force of a ship and stabilize the straight advance property by providing at least one variable-width means optionally expanding the lateral width of a rudder plate at least one side of the rear face area of the rudder plate.

CONSTITUTION: In a rudder plate 3 installed in swaying manner horizontally behind the screw propeller 7 of a ship 1 with a rudder head shaft 2 serving as a suspending supporter, the rudder plate 3 itself is formed into a streamline shape similar to the plan view of a fish with no caudal fin. Oblong rectangular means storage windows 4 are formed on both sides of the rear face area of the rudder plate 3, and variable-width means 5 are openably fitted to the means storage windows 4 via fulcrum mechanisms 6 with a hinge structure. When the ship 1 cruises in a harbor at a low speed or in the light draft state, the variable-width means 5 are protruded to both sides of the rudder plate 3, thus opposite turning forces are converted into the straight advance property of the rudder plate 3 by the resistance of fluids F applied to the variable-width means 5 on both sides, the stability of the control attitude of the rudder plate 3 itself is increased, and the drift of a hull is dissolved.