A mass spectrometer having a resolution improved by introducing ions into a mass spectrometry part with a high efficiency is provided with a small-sized, simple configuration. The mass spectrometer includes an opening/closing mechanism provided between a sample introducing piping part for introducing a sample into the mass spectrometry part and the mass spectrometry part to conduct gas introduction intermittently and control sample passage. The mass spectrometer further includes a pump mechanism to evacuate a high pressure side of the sample introducing piping part, that is, an opposite side of the opening/closing mechanism to the mass spectrometry part to have a pressure in a range of 100 to 10,000 Pa.