In this paper, we prove that the center of buoyancy of a ship is equal to the center of hydrostatic pressure. This subject is an unsolved problem in physics and naval architecture, even though the buoyancy taught by Archimedes' principle can be obtained clearly by the surface integral of hydrostatic pressure. Then we thought that the reason why the vertical position of the center of pressure could not be determined was that the horizontal force would be zero due to equilibrium in the upright state.
 As a breakthrough, we dared to assume the left-right asymmetric pressure field by inclining the ship with heel angle. In that state, the force and moment due to hydrostatic pressure were calculated correctly with respect to the tilted coordinate system fixed to the floating body. By doing so, we succeeded in determining the center of pressure.
 Then, by setting the heel angle to zero, it was proved that the center of hydrostatic pressure is equal to the well-known center of buoyancy, i.e., the centroid of the cross-sectional area under the water surface. Specifically, the above proof is first shown for a rectangular cross-section, and then for an arbitrary shape of floating body by applying Gauss's integral theorem. And we show an extension to the center of buoyancy for a 3-D floating body.