The robust structural optimization Design of a plate wing is investigated considering the bifurcation in higher mode flutter, which affects the robustness of an optimum design and the convergence of an optimization. To improve the critical dynamic pressure of supersonic flutter under the constant total mass, the thickness distribution of a delta wing is considered as design variables. The constraints are proposed of keeping eigenvalues between adjacent modes apart by the specified distance which is composed of the sensitivity of the distance of eigenvalues. The numerical results show the improvement of robustness of the optimum design.