The dynamic behaviour of a vertical circular cylinder exerted by wave forces is discussed on the basis of the calculation of wave making resistance.
　In Part 1, the effect of the relative motion on the dynamic response of an elastic pile is examined. A pile is replaced by a system having one degree of freedom, with an equivalent mass and load concentrated at the top. Some examples of calculation suggest the importance of the wave making resistance as a damping force rather than the added mass.
　In Part2, the wave force on a fixed circular floater and the wave making resistance due to the horizontal, vertical and rotationary oscillation of a floater are approximately cilculated and the equations of motion are obtained. The motion of a circular floater under simple restraint can be solved from these equations.