A newly developed mass spectrometer which employs a sputter ion pump was flown on a balloon in May, 1975, and obtained mass spectra for neutral constituents in the stratosphere. The measured mass spectra show an abnormally high concentration at the mass number 64, to be about 30 ppm. This particular component has a similar altitude profile to that of the aerosols, and could be assigned to sulfur dioxide. Such a high concentration of SO2 that was measured in this flight may be partly associated with the eruption of Mt. Fuego.