A source gas introduction line for introducing source gas containing CO2, a first membrane separator for membrane-separating CO2 from source gas, a first permeable gas discharge line for discharging first permeable gas permeated by membrane separation of the first membrane separator, a first non-permeable gas discharge line for discharging first non-permeable gas not permeated by membrane separation of the first membrane separator, a second membrane separator provided at a downstream side of the first membrane separator and for further membrane-separating CO2 from the first non-permeable gas, a second permeable gas discharge line for discharging second permeable gas permeated by membrane separation of the second membrane separator, a second permeable gas return line branched from a part of the second permeable gas discharge line and for returning the second permeable gas to a source gas side, and a CO2 concentration meter are included.