A real elastic motion of a very large floating structure is complicated because a field that a VLFS will be installed is random sea area. Then, it is important to predict horizontal motions too, because slowly varying drift motion may occur in irregular waves. So, the motion responses must be investigated in irregular waves. In this paper, one method in order to calculate a time series of wave drift forces is shown. The validity of that method and the estimation method of wave drift forces is confirmed by comparing with the corresponding experimental results. Then, the time histories of vertical and horizontal displacements are compared. And, this paper proposes the some examinations on the estimation method of the wave drift force. They are the effect of an interaction among directional angles of incident waves and the necessary term in order to predict it.