The purpose of the present study is to investigate how longitudinal stiffener plates can improve plastic deformation capacity of the H-section steel beams with a thinner web. Efficiency of the retrofit can be quantitatively estimated by an accumulated plastic deformation obtained from cyclic loading tests and numerical simulation. The quantitative estimate method of prevention effect of local buckling by the theory of elastic buckling of plates is proposed. The design procedure of stiffeners by prediction formula of plastic deformation capacity and increasing rate in strength is proposed.