PURPOSE: To prevent a lateral tilting angle of a hull due to sudden steerage from becoming large, by a method wherein an auxiliary rudder is provided below a main rudder, a tilting angle of the hull is detected and the auxiliary rudder is operated based on the detected value and a rudder angle signal of a main rudder.

CONSTITUTION: An auxiliary rudder 7 whose area is smaller than that of a main rudder is provided below the main rudder 3 and a main rudder controller 6, an auxiliary rudder controller 9 and a lateral tilting angle detector 10 of a hull are provided. A rudder angle signal of the main rudder controller 6 and a lateral tilting angle signal of the lateral tilting angle detector are applied to a controller 11, a rudder angle of the auxiliary rudder 7 is made identical with that of the main rudder 8 when a lateral tilting angle is within a range of a predetermined value, the auxiliary rudder 7 is operated in a direction reducing the lateral tilting angle irrespective of the rudder angle of the main rudder 3 when the lateral tilting angle exceeds a safety tilting angle, and the lateral tilting angle is reduced by operating the main rudder 3 in the reverse direction when the tilting angle arrives at a critical tilting angle further. With this construction, the lateral tilting angle of the hull resulting from sudden steerage is prevented from becoming large and the hull is prevented from overturning.