In the previous report, the authors clarified the existence of the minimum stiffness ratio, γ ^U _min, of a stiffener against the ultimate strength of a stiffened plate in similar to the buckling strength, γ ^B _min. In this report, elastic large deflection analysis is performed on the stiffened plate, and the phenomenon of the secondary buckling after the primary buckling is indicated. Then, a series of experiment is conducted to confirm the phenomena predicted by the theory. From these theoretical and experimental results, an approximate analytical method to determine γ ^U _min is newly proposed as the intersection of the secondary buckling strength curve and ultimate strength curve of a locally failured plate, when there exists no initial deflection. When there exists initial deflection, the effective minimum stiffness ratio _effγ ^U _min is defined as the intersection of a curve of the initial yielding of bending stress of a stiffener and that of membrane stress of a plate. This method can also be applied to determine γ^U_min of a stiffened plate with arbitary aspect ratio and arbitary number of stiffeners. The effect of welding residual stresses on the buckling and ultimate strength is also discussed, together with on γ^U_min and _effγ ^U _min and the minimum stiffness ratio against the buckling strength, γ ^B _min.