To extract nitrogen and/or helium by liquefying only a specified compound from a compound containing the specified compound and another compound having lower boiling point available under a specified pressure.

Natural gas extracted through a line 4 is expanded through an expander 5, a liquid phase turbine at the inlet and a gas-liquid phase turbine at the outlet until a pressure low enough to evaporate the majority of nitrogen is attained. The expanded natural gas is fed through a line 6 to a contactor C1 and, at the outlet thereof, a first flow F1 principally comprising liquid-phase methane at the foaming point is discharged through a line 8 while a second flow F2 comprising methane, nitrogen and/or helium is discharged through a line 9. Methane is condensed when the second flow F2 flows through a condenser and then it is separated from nitrogen and/or helium in a separation drum S.