Usually we have considered the relative motions in the case that both motions are rectilinear or one is rectilinear and the other is circular. And the method taken so far has been Vector Calculation or the analysis using the time variable as the parametric representation. In this paper, relative motions against general curvilinear motions are universally considered dy using Cesaro's Natural Geometry. It is manifested that the relative motion which the circular motion works out against the rectilinear motion is Trochoidal motion and other several results are obtained. In the last part, we take up several exsamples indicating that the above method is very valid and uniform.
