Offshor structures should have a high degree of structural safety even under extreme environmental loadings. The authors have developed a composite steel-concrete structure of sandwich system for the strength members of huge offshore structures, where concrete is placed between steel plates. In the previous paper, the authors carried out both experimental and theoretical investigations into the strength of the composite structure, It was clarified that the composite structure has various excellent properties : The ultimate load-bearing capacity of the composite structure is very high and it can absorb a great deal of energy until failure under any type of loading conditions. The composite structure exhibits nonlinear behavior before its ultimate strength, caused by the following phenomena; 1) cracking of concrete under tension, 2) plastification of concrete under compression, 3) plastification of steel plate and 4) gap between steel plate and concrete. In the present paper, the authors developed a method of the nonlinear analysis using the finite element method. In the analysis, the material nonlinearities of both concrete and steel are fully taken into account as well as the nonlinearity caused by the gap between steel and concrete. The nonlinear analysis accurately represented the behavior of the composite structure under tests.