In the near future, various huge off-shore structures will be constructed. It was concluded through preliminary studies that composite steel-concrete structures are best fitted for the strength members of the huge structures, due to the high strength and ductility of steel and due to the low ratio of the cost to the weight and reliability of concrete. In the paper, the authors carried out both experimental and theoretical investigations into the strength of the composite steel-concrete structures where concrete is placed between steel plates. Experiments were carried out using the two-dimensional models of the composite structures under bending, shear and combined bending and shear. The fundamental properties of the composite structures were clarified through the experiments. A new method for the ultimate strength analysis of the composite structure was developed. The calculated results were in good agreements with experimental ones. The effects of the depth of the composite structure, the sectional areas of compression and tension steel plates and the compressive strength of concrete upon the ultimate strength were investigated by the proposed method of analysis. It was clarified that the depth of the structure and the sectional area of the steel plate under tension exert decisive influences upon the ultimate strength.