Abstract Stability tests of two quadrupole mass spectrometers (QMSs) were performed using a two-stage flow-dividing system from the following viewpoints: (1) fluctuation and drift of ion current, (2) repeatability of ion current, (3) change in ion current owing to prior conditions of use, (4) long-term stability of sensitivity, and (5) interference effect. These tests were performed at the pressure from 8 × 10 −6 Pa to 8 × 10 −4 Pa using N 2 , Ar, He, and H 2 . The fluctuation and drift of two QMSs over 1 h were within 2%. The repeatabilities at pressures of 8 × 10 −6 Pa and 8 × 10 −5 Pa were within 2% during eight cycles with an interval of 1 h between each cycle. However, the repeatability at 8 × 10 −4 Pa increased to be within 4%. The changes in sensitivities of QMS-1 and QMS-2 for 757 days were less than ±15% and less than ±25%, respectively. The change owing to the interference effect was less than 2% while the partial pressure of the interference gas was under 2 × 10 −4 Pa. However, both positive and negative changes were observed less than 60% with increasing the partial pressure of the interference gas until 8 × 10 −3 Pa.