The hydroelastic response analysis of a mega-float structure in waves is usually made by integrating its structure model and fluid model, but usually the analysis model become a very large one. It is, therefore, difficult from the aspect of calculation time limitation to make such an analysis by exactly incorporating the structurally discontinuous parts such as girders with large openings in a mega-float into the above structure model. On the other hand, since the mega-float structure has many structurally discontinuous parts, it is very important to establish an evaluation method of the stresses caused by the hydroelastic response at such parts. In these circumstances, noticing the fact that wave-exciting forces acting on the mega-float are concentrated at the end of the wave incident side, the authors have invented such a stress analysis method of mega-float structurally discontinuous parts (girders with openings, etc.) as is mentioned below and verified its propriety by actual application of it. The method is that a mega-float structure is replaced by an FEM vibration model of a 3-dimensional slip-shaped girder supported by a buoyancy spring which is excited at the end of the wave incident side in the vertical direction and the vibration response analysis is carried out.