A high-resolution quadrupole mass spectrometer (QMS) which can detect He-3 in HD and He-4 in D2 has 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 of He-3/HD and He-4/D2 were 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.