It is necessary to estimate slowly varying drift forces on very large floating structures (VLFS) for their safety. The aim of our research is to develop an estimation method of such forces on VLFS in the real seas, i.e. in multidirectional irregular waves. We knew such a method for rigid bodies, but it seems to be complicated to extend the method for VLFS directly because of their hydroelastic behavior. Thus, in the previous report, we presented an easy but legitimate estimation method in which we can evaluate slowly varying drift forces by means of the integration of the squared relative wave height and also we validated our method with model tests in long crested irregular waves. In this 2nd report, we carried out a numerical calculation with various flexural rigidities of VLFS, incident angles, and incident wave periods to validate our method more generally. We also validated our method with model tests in short crested irregular waves so that we can demonstrate our method can be applied for VLFS in multidirectional irregular waves.