As one of the characteristics of ship hull structure, many holes are made on web plates for access and weight saving. Many studies have already been made to investigate the declining of the buckling strength by the perforation. Nevetheless, the shear buckling failure of such a structure has not ceased to exist. On this paper the instability of perforated plates having reinforcement is studied by means of FEM analysis and experimentation to determine the effect of stiffenings on the buckling strength. This study mainly deals with the simply supported plate with an oval hole subjected to shearing force. As for the reinforcing method, stiffener, spigot-plate and doubling are chosen. The conclusions drawn are as follows; 1) As the reinforcing method, stiffener is most desirable in respect of elastic range. 2) On the presumption of the representative ratio of thickness to breadth for the web plate, yielding will take place before the buckling failure. 3) In case of the above, stiffener does not improve much the buckling strength and it is better to reinforce the perforated plate by doubling.