Disclosed is an apparatus for controlling a water plane area capable of preventing a marine structure from being inclined by controlling a water plane area of a column part, and controlling a water plane area of the column part by partially controlling the diameter of the column part by injecting or discharging fluid. The apparatus for controlling a water plane area comprises: a column part supporting a marine structure and having a groove; a space forming unit disposed to cover the groove and forming a space between a side surface of the column with the groove and an inner surface; and a fluid injecting unit controlling the volume of the space by injecting or discharging a fluid from the space.
