Two important factors disturbing the accurate measurement of partial pressures by a quadruple mass spectrometer (QMS) are described in this paper. One is the effect of ions desorbed from the surface of ionizer by electron stimulated desorption (ESD). The conventional QMS cannot separate the ESD ions from the gas phase ions produced in the ionizer. The other one is the background noise due to the irradiation of the secondary electron multiplier (SEM) by the X-rays and the vacuum ultra violet (VUV) photons generated in the ionizer. The new QMS with the bessel-box type energy analyzed is proposed for the reduction of those effects.