  We extend our previously developed method, "structure integration", to
evaluate free energy directly for magnetic large systems on given lattice. The
present method express the density of states(DOS) as the parameters independent
of system but depend on lattice, and replace the DOS unknown a priori as that
known a priori. Through two-dimensional square lattice Ising model, we find
that the present method can evaluate magnetic free energy efficiently and
accurately above critical temperature without iterative method like Monte Carlo
simulation.
