In order to estimate the concentrations of atmospheric nitrous oxide (N₂O) during the last 250 years, air samples were extracted from an Antarctic ice core, H15, using a dry extraction system and were then analyzed with a precision of ±2 ppbv. The results obtained were clearly less scattered and much tighter than those of the previous studies. Our data showed that the concentrations of atmospheric N₂O in the 18th century were about 276 ppbv on average. It was also obvious that the N₂O concentration began to increase in the mid‐19th century and reached approximately 293 ppbv around 1965, the trend of the concentration increase correlating quite well with the direct atmospheric measurements at the South Pole. Such an increase in the atmospheric N₂O concentration is thought to be of anthropogenic origin.