The dog stays, consisting of a short stay bolt and a saddle shaped spacer having three legs, are often required for the reinforcement of the flat plate portions of pressure vessels when the ordinary stays are inapplicable due to the structural restriction of the vessels. The authors, carring out a series of experiments, could establish the designing formulae for the stay bolt and moreover make the following matters clear. (1) The stresses on the flat plate are decreased proportionately with the ratio of the legstretch circle to the periphery circle, except the stress concentration caused by the contact of legs to the plate. (2) The effect of the Stay for restricting deformation is very noticieable and proportional to the ratio mentioned above. (3) The diameter of the stay bolt can be so calculated as to support the one fourth of the area to be reinforced when the periphery is fixed.