Nitrogen is considered to have been a major component of terrestrial atmosphere over the long periods of geological time. The isotopic composition of ancient atmosphere, if obtained, may provide rigid constraints on models of atmospheric evolution. However there are few data available in literature. Chert is a candidate for this purpose possibly to trap some aliquot of paleoatmosphere. We report here argon and nitrogen abundances and nitrogen isotope ratios in six chert samples with various formation ages using stepped heating and static mass spectrometry. Results indicate that the argon in the samples has multi-component signatures. Nitrogen in the lower temperature release with the maximum argon release gives relatively constant δ¹⁵N values of around 0‰ while that of the highest step shows variable δ¹⁵N values from about 0 to + 12‰. The N₂/Ar ratios of the lower step are significantly constant compared with those of the highest step. If the lower step nitrogen is trapping and reflecting the ancient atmosphere, the atmospheric δ¹⁵N value has been constant over the period of 3 b.y.