A high‐resolution quadrupole mass spectrometer (QMS) which can detect3He in HD and4He in D2has been developed. The QMS is equipped with two resolving powers, i.e., a high‐resolution mode and a normal one, and voltages for realizing the two modes are supplied by a common circuit. The high‐resolution mode adopts a condition of the second stability zone (zone II) in the Mathieu diagram and can analyze a range of 1–9 amu. The detectable peak ratios of3He/HD and4He/D2were obtained at roughly 0.1 and 10−4, respectively. The normal mode adopts the first stability zone (zone I) and has an analyzable range of 1–60 amu.