The quadrupole mass spectrometer (QMS) with the Bessel-Box type energy analyzer has been developed. The analyzer is placed between the ionizer and the quadrupole mass filter for the separation of the gas phase ions and the electron stimulated desorption (ESD) ions produced in the ionizer according to their kinetic energies. Three important results for the improvement of the accuracy of partial pressure measurements have been obtained by the QMS. First, the elimination of ESD ions gives the simple mass spectra of gas phase ions for the accurate analysis of the spectra. Second, the signal intensity profiles of the peaks of parent ions and the fragment ions with same mass to charge ratio (Mle) in the mass spectra are obtained and are discussed in relation to the mechanism of ion formation. Third, the sensitivity of the QMS for the very small amount of impurities in the gases of higher pressure (high vacuum region) is also improved down to a few tens of parts per billion of the total pressure due to the reduction of background noise. The noise is much reduced by the geometrical structure of the energy analyzer which prevents the ion collector, a secondary electron multiplier, from the irradiations of vacuum ultraviolet photons and soft x rays generated in the ionizer.