[en] I has been known that selective oxidation occurs on superalloys in the HTGR (High Temperature Gas-cooled Reactor) helium environment and the oxide scale is mainly composed of chromium, manganese, titanium and aluminum oxide. However, the relationship between the composition of the oxide scale and the base metal in the vicinity of grain boundaries has not been well established. In this paper the authors investigated the oxidation behavior of Incoloy 800, paying close attention to the distribution of alloying elements at grain boundaries of the base metal. (Auth.)