Provided is a mass spectrometer including a vacuum chamber containing a cell used for causing ions originating from a sample to come in contact with a predetermined gas supplied from a gas supplier (20), which includes a three-way valve (204) configured to selectively send the gas into either a first passage (22) for sending the predetermined gas to the cell or a second passage (23) for discharging the gas into vacuum state. The mass spectrometer also includes a controller configured to control the three-way valve so as to send the gas into the second passage during an analysis period in which ions are not caused to come in contact with the gas within the cell and/or during a non-analyzing period, as well as to send the gas into the first passage during an analysis period in which ions are caused to come in contact with the gas within the cell.