Plate buckling preventions of steel plate panels are keys to enhance seismic performance of existing steel bridges and additional stiffeners can improve their buckling strength and ductility. Then focused on in this study is a seismic retrofitting technique of steel plate panels with lack of ductility using an L-shaped member attached with high strength bolts. Also focused on is the influence of an initial stress on the strengthening effects. 5 short column specimens with steel hollow cross section before/after being strengthened are used in compression tests. It is concluded that the bolted additional stiffeners as well as the welded ones effectively work for buckling prevention of steel plate panels.