Provided is an apparatus for reducing the pitching and rolling motion of a vessel, the apparatus including a plurality of buoyancy members spaced apart from each other and vertically slidably mounted on a vessel, a plurality of hydraulic cylinders that are connected to the upper end portions of the respective buoyancy members and receive the locomotive power of the buoyancy members that rise from waves, and a hydraulic connection line provided in such a manner that the hydraulic pressures of the hydraulic cylinders are interconnected, so that the hydraulic pressures, which are generated from the rise of the hydraulic cylinders due to the rise of the buoyancy members located at the peaks of the waves, can be supplied to the hydraulic cylinders located at the troughs of the waves to cause the buoyancy members to fall, thereby increasing the buoyancy of the vessel and restraining the pitching and rolling thereof.