It is well known that the fracture toughness of a crack in a thin adhesive layer constrained by hard adherends depends on the thickness of the adhesive layer; however, the precise mechanism of the dependence has not yet been elucidated. In our previous study, we investigated the stress distribution around a crack tip in an adhesive layer using the finite element method (FEM), we discovered the presence of higher hydrostatic stress around a crack tip in thinner adhesive layers. In this study, the damage around crack tips in thin adhesive layers of rubber-modified epoxy resin is analyzed using the FEM in conjunction with Gurson's model, which can well describe the yielding of porous material such as rubber-modified epoxy resin. The decrease of the fracture energy with the decrease of the bond thickness can be estimated by this analysis, but it is not clear why the maximum fracture energy occurs at around 0.4 mm of bound thickness.