In the previous reports, the authors have investigated the detailed behavior of stiffened plates under thrust, and maintained the concept of the minimum stiffness ratio of the stiffener, γ ^U _min, for the compressive ultimate strength of stiffened plates. In this paper, a theoretical investigation into the ultimate strength of a girder with a horizontal stiffener under bending is performed, and it is clarified that there exists the minimum stiffness ratio, γ ^U _min, for the ultimate strength of a girder under bending in similar to the compressive ultimate strength of stiffened plates. Furthermore, a simple approximate method is proposed to predict the ultimate strength of a girder with a horizontal stiffener under bending, and the relevant minimum stiffness ratio, γ ^U _min.