Recently we developed an analyzer capable of measuring hydrogen concentrations of less than 5 cm3/1000 cm3. The analyzer is characterized by the fact that it uses a carrier gas purifier, a thermal conductivity detector, high- and low-temperature separation columns, and an “after-cut method”to remove impurities from sample and carrier gases. In the after-cut method, nitrogen gas is removed by switching gas lines. In the analyzer, a trace reduction detector is also used to detect extremely small amounts of hydrogen based on the reduction reaction of mercuric oxide with hydrogen. This is another distinctive characteristic of this analyzer. In the present study, we evaluated the performance of the analyzer in measuring deuterium concentrations. We measured spectra for five sample gases with different concentrations and examined the peaks and retention times of the spectra. The peaks and retention times were stable within a relative standard deviation of less than 10%, and a definite linear relationship between the peaks and deuterium concentrations was also observed. Based on these results, we have considered how to develop an analyzer capable of directly measuring the deuterium existing in natural air.