In this paper the authors investigate the vortex-making damping forces of a column with a circular footing, varying its depth. Further, the motions for one model in waves are measured and compared with those estimated by a linear potential theory. It is found that the amplitude of the heaving motion is smaller than the linear calculation in the resonant oscillation, but in contrast it becomes finite at the wave-excitationless frequency. In order to improve such estimations, the vortex-making effect is taken into consideration not only in the damping forces but also in the wave-excitation forces. The linearised vortexmaking damping coefficient is constant for the smaller amplitude of oscillation, and is quite larger than the wave-making damping coefficient. Further, it increases with the amplitude when the amplitude ratio becomes greater than a certain value. Therefore, the heaving motion in resonance reduces very much and the calculation shows a good coincidence with the experiment. In addition, it is assumed that the wave-excitation force is also affected by the vortex-shedding at the footing. This effect becomes important at the wave-excitationless frequency where the excitation force may otherwise be zero, and the finite heaving motion can be well explained.