The equation derived by Kotani and King relating to the excess pressure valid to the second order in the acoustic field is examined. It is shown that the solution of the usual wave equation can be used in the case when the radiation pressure in the Langevin's sense is derived from this equation. The reason as to why not only the normal force but also the tangential force on the nonstiff surface result due to the radiation pressure defined from the viewpoint of the pressure in hydrodynamics, is clarified by considering the motion of the surface. The general expression for the radiation pressure on the rigid object which is in motion due to inertia effect, is led by introducing moving coordinates.