In a cross section of a beam where no shear force exists the normal stress distrilution is rectangular at the fully plastic state under the assumption of perfect plasticity of material. Under the existence of shear force the normal and stress distributions are somewhat complicated which must satisfy conditions of equilibrium, yielding and compatibility. By approximating these distributions of normal and shear stress by rectangular ones which satisfy yield condition, rational results are obtainable. In this paper the interaction between the plastic moment and shear in I-section beams with cutouts is treated under the assumption of rectangular distributions of normal and shear stress and the interaction curves between the shearing-bending ratio r = S / S ₀/ M / M ₀ and the plastic moment Si Mo coefficient m = M / M ₀, at several opening ratios of cutouts, are obtained concerning three types of I-section. And using these interaction curves, the equistrength opening ratio κ, within the bounds of which the cutout dose not at all weak down the collapsing strength of the beam, is obtained as the function of longitudinal position on a few beams with typical end condition and load condition.