[en] Air samples were collected at the two points in the large marshland of Western Siberia lowlands. Methane in the sample was determined by a carbon isotope ratio analysis. 80% of methane in air of Surgut area, where natural marshland and oil plant were distributed, was generated from fossil in both summer and winter season. In summer season, methane generated from microorganism except fossil was determined by δ¹³C value. The value was the same as the previous result. This result is agreed with the geographical feature of Western Siberia. These facts proved that this technology was useful measurement for analysis of methane generated from fossil or marshland in the Western Siberia Lowland. (S.Y.)