a mass spectroscope includes an ion source for generating ions , a mass spectrometry portion , and an ion retention portion arranged between the ion source and mass spectrometry portion for storing , cooling and / or dissociating the ions before the ions are discharged into the mass spectrometry portion . the mass spectroscope further includes a flow adjusting device for adjusting a gas flowing into the ion retention portion from an outside , and a control device for controlling the flow adjusting device in order to appropriately set a gas pressure in the ion retention portion according to an operation mode such as an introducing operation , a retention operation , and a discharging operation .