This study is to establish a method to calculate an instable structure in nonlinear structual analysis. In case stiffness matrix is approximately singular or not regular, derive the optimum approximate solution for a stiffness equation by decomposing the stiffness matrix with the singular value, and then proceed with analysis. A resultant error will be dissolved by convergence calculation. On the other hand, to calculate an instable phenomenon resulting from the discontinuity of the mechanical behavior of a structural member, analysis will be proceeded while choosing such a loading route as not produce instability. This method is an improved version of ordinary nonlinear finite element method. Several analyses have resulted to verify that this method developped in this paper is markedly effective in analysis an instable structure.