The paper is based on some of the results of an investigation of natural gas in Japan as conducted under the direction of the Aeronautical Research Institute of the Tokyo Imperial University. The author is concerned with the variation in chemical composition of natural gas with its modes of geologic occurrence. In general natural gases are classified as (1) those of igneous or inorganic origin and (2) those of sedimentary or organic origin. The natural gases from Quaternary formations contain mainly methane as the hydrocarbon compound, but the oil-bearing Tertiary formations contain not only methane, but also other hydrocarbon compounds. Some natural gases from the oil-bearing formations appear to show chemical variation with geologic conditions. Helium content of sedimen ary natural gases from Quaternary and Tertiary formations is generally negligible. Several tables of chemical analysis accompany the paper.