*********** 102391B.ENG *********** ABSTRACT from NKK Technical Report No. 1991, no 136 p. 61-68, Japanese Text with 14 Figures, 5 photos and 4 tables in English lnvestigation of Dai-ichi Ryogoku Bridge Made of Unpainted Weathering Steel Which has been Spanned for 20 Years Over Sea lsamu Kanoh, Akihiro Tamada, Kenji Kato, Hidetoshi Hara and Yoshikazu Saito Dai-ichi Ryogoku Bridge, one of the oldest bridge made of weathering steel, has spanned over sea for 20 yearts. Investigations of rust layer formed on the surface by outer appearance, microscope and com- position analysis on cross section and chloride content ac- cumulated on it was carried out. The results revealed that stabel and protective rust layer with uncrystalized alloy rich inner layer is formed on the surface. The results also revealed that the chloride content in the layer was extremely low. As a conclusion, the application of the weathering steel to the bridge over sea can be permissible if the weathering condition is moderate.