A mass spectrometer comprising: a mass spectrometry part for conducting mass spectrometry on a sample; a sample introducing piping part for introducing a sample into said mass spectrometry part; an opening/closing mechanism provided between said sample introducing piping part and said mass spectrometry part to open/close thereby to control passage of said sample; an opening/closing control part for controlling said opening/closing mechanism; a first pump for evacuating an opposite side region of said opening/closing mechanism to said mass spectrometry part to have a pressure of 100 Pa or greater and 10,000 Pa or less; and an evacuation pipe for connecting said first pump and said sample introducing piping part, wherein: said opening/closing mechanism comprises a movable member and a space within which said movable member is movable; said space comprises an opening part to said sample introducing piping part, an opening part to said mass spectrometry part, and an opening part to said evacuation pipe; said opening/closing control part is adapted to control said movable member, when passing said sample, to open a passage between said opening part to said sample introducing piping part and said opening part to said mass spectrometry part, and said opening/closing control part is adapted to control said movable member, when not passing said sample, to close a passage between said opening part to said sample introducing piping part and said opening part to said mass spectrometry part and open a passage between said opening part to said sample introducing piping part and said opening part to said evacuation pipe, characterized in that said opening/closing control part is adapted to control said movable member, when passing said sample, to close a passage between said opening part to said sample introducing piping part and said opening part to said evacuation pipe. a mass spectrometry part for conducting mass spectrometry on a sample; a sample introducing piping part for introducing a sample into said mass spectrometry part; an opening/closing mechanism provided between said sample introducing piping part and said mass spectrometry part to open/close thereby to control passage of said sample; an opening/closing control part for controlling said opening/closing mechanism; a first pump for evacuating an opposite side region of said opening/closing mechanism to said mass spectrometry part to have a pressure of 100 Pa or greater and 10,000 Pa or less; and an evacuation pipe for connecting said first pump and said sample introducing piping part, wherein: said opening/closing mechanism comprises a movable member and a space within which said movable member is movable; said space comprises an opening part to said sample introducing piping part, an opening part to said mass spectrometry part, and an opening part to said evacuation pipe; said opening/closing control part is adapted to control said movable member, when passing said sample, to open a passage between said opening part to said sample introducing piping part and said opening part to said mass spectrometry part, and said opening/closing control part is adapted to control said movable member, when not passing said sample, to close a passage between said opening part to said sample introducing piping part and said opening part to said mass spectrometry part and open a passage between said opening part to said sample introducing piping part and said opening part to said evacuation pipe, characterized in that said opening/closing control part is adapted to control said movable member, when passing said sample, to close a passage between said opening part to said sample introducing piping part and said opening part to said evacuation pipe. said opening/closing mechanism comprises a movable member and a space within which said movable member is movable; said space comprises an opening part to said sample introducing piping part, an opening part to said mass spectrometry part, and an opening part to said evacuation pipe; said opening/closing control part is adapted to control said movable member, when passing said sample, to open a passage between said opening part to said sample introducing piping part and said opening part to said mass spectrometry part, and said opening/closing control part is adapted to control said movable member, when not passing said sample, to close a passage between said opening part to said sample introducing piping part and said opening part to said mass spectrometry part and open a passage between said opening part to said sample introducing piping part and said opening part to said evacuation pipe, characterized in that said opening/closing control part is adapted to control said movable member, when passing said sample, to close a passage between said opening part to said sample introducing piping part and said opening part to said evacuation pipe.