Possibility to utilize sea areas by using very large floating structures (VLFSs) has remained when we consider impact to ocean environment, a change of uses, decommission and lifecycle. We can consider installing VLFSs in very shallow seas as well as deep sea areas. In Japanese case, it is, for instance, in Tokyo bay areas. Although added-mass increases very much due to a huge horizontal area, studies in which effects of variation of the added-mass on hydroelasticity are summarized are not a lot. Besides, how the elastic deformation changes has not also been investigated when clearance between seabed and a bottom plan decreases because of draft increasing in shallow sea areas. This study used the linear potential theory bases prediction method for hydroelasticity problems. The paper calculated pontoon type VLFSs. Then it was investigated and summarized how the elastic motion characteristics were affected by water depth, mass of a VLFS and the clearance and in taking into account of variation of the added-mass. From the results, effects of physical draft of VLFSs was not large when deep water depth and we could calculate the elastic response only using suitable mass of a VLFS even if the draft of the VLFS to be calculated was a bit different. In addition, when the clearance decreased, the added-mass increased very much and characteristics of the elastic motion were affected very much.