It is well known that there exists the hydrodynamic interaction between ship's hull and rudder which may have a remarkable effect on ship maneuvering. By many experimental investigations of lateral force acting on a ship during maneuver, it is found that the hydrodynamic force which is induced on the main hull of a ship by steering its rudder comes up to about 30% of rudder force. At present, however, the effect of presence of main hull on the rudder force is not yet made clear. The main aim of this paper is to deal with the latter problem. Assuming that the hull and the rudder can be replaced by a low aspect ratio rectangular wing and its flap which is separated from the wing, the integral equations based on nonlinear lifting surface theory are derived, and the results of numerical calculation are compared with the experimental data. The calculation values of normal force acting on rudder and main hull agree fairly well with the experimental ones. Furthermore, by comparing tine hydrodynamic forces of rudder and main hull obtained in hull-rudder system with those obtained when rudder and main hull exist independently, the authors discuss the hull to rudder interaction. Consequently, it is shown that additional hydrodynamic force due to steering is induced on the main hull, but on the contrary, the rudder effectiveness is reduced compared with that in open water.