Two series of residual gas analysis were made by using a quadrupole mass filter for two independent vacuum systems. One of the systems comprises a oil diffusion pump (4″)using silicone DC-705 as a working fluid, and the other two oil diffusion pumps (2.5″) using naphthyl derivative (mono eicosyl). The results of the analysis indicate the existence of an unnegligible amount of the diffusion oil vapor in the vacuum chamber. The two diffusion pump oils gave their own characteristic mass peaks, and tentative assignment was made for the structures of these fragment ions. It was found from the residual. gas analysis that the vacuum system of the naphthyl derivative diffusion pumps is more favorable for carrying out some kind of electron impact experiment.