So far there are few papers in which the finite element method has been studied for analyzing “built-up plate structures”, in which in-plane and out-of-plane actions have to be considered simultaneously. For this kind of problems, the authors develop a new approach using the finite element method in this paper, where the principle of minimum potential energy is adopted for in-plane action and Hellinger-Reissner's principle for out-of-plane action. The displacement function in this calculation, is as follows ; { u v w M_x M_y M_xy } = [1 _xy 1 _xy 1 _xy 111] {α₁α₂………α₁₂} By using this new method, several kind of structures are analyzed ; i) A simply supported and a clamped square plates subjected to uniformly distributed lateral pressure. ii) A canti-lever beam with wide face plate. iii) A transversely and longitudinally stiffened plate subjected to uniformly distributed lateral pressure. iv) A corrugated plate subjected to lateral load. v) A corrugated plate under in-plane shear deformation. As to the accuracy of this analysis, in the case of a simply supported square plate subjected to uniformly distributed lateral pressure, percent error of 400 elements solution for Timoshenko's series solution is about 2% with respect to the maximum deflection and about 0.5% with respect to the maximum bending moment. It is found that this method is very useful for the analysis of “built-up plate structures”.