NOVELTY - The apparatus has membrane separator (1) that is provided to separate membrane separation waste gas (14) to which nitrogen concentration is raised and membrane separation process gas (13) containing residual nitrogen. The nitrogen is equipped with an adsorption agent adsorbed in adsorption tank as receive membrane separation process gas from first chamber (1A). A pressure-swing-adsorption apparatus (2) is provided to produce product gas in which methane concentration is raised, and the waste gas in which nitrogen concentration is raised, and containing methane.    USE - Apparatus for separating nitrogen from natural gas.    ADVANTAGE - The nitrogen which remains after membrane separator separates nitrogen is adsorbed with pressure-swing adsorption apparatus, so that the source gas reduces nitrogen concentration to medium level with membrane separation processing apparatus. Thus the nitrogen concentration is reduced to ultra-low concentration with pressure swing adsorption apparatus. Therefore the product gas with raised nitrogen residual concentration and reduced methane concentration is obtained. The high recovery of methane is obtained. The pressure drop is small and the usage amount of membrane is reduced. The usage-amount of adsorption agent is suppressed and the cost of adsorption agent is reduced.    DETAILED DESCRIPTION - The return supply of portion of waste gas is carried out to pressure-swing adsorption apparatus, and remainder is discharged. A membrane separator is provided with first membrane separator and second membrane separator. Two chambers are divided by the separation membrane. The source gas is supplied to the first chamber of first membrane separator while the pressure-swing-adsorption apparatus is connected to first membrane separator so that return supply of the medium |pressure waste gas is carried out to first chamber of first membrane separator. The pressure-swing-adsorption apparatus is connected to second membrane separator so that the low pressure waste gas is vacuum-sucked, and nitrogen concentration is raised. The second chamber of first membrane separator is connected to the first chamber of second membrane separator. The second chamber of first membrane is connected to second membrane separator so that the low pressure waste gas is merged and the primary membrane separation waste gas from the second chamber of first membrane separator is supplied to the first chamber of second membrane separator. The primary membrane separation process gas from first chamber of first membrane separator is merged, and the secondary membrane separation process gas from first chamber of second membrane separator is supplied to pressure swing- adsorption apparatus. The secondary membrane separation waste gas is discharged from second chamber of second membrane separator. An INDEPENDENT CLAIM is included for a nitrogen separation method which involves separating membrane separation waste gas to which nitrogen concentration is raised and membrane separation process gas containing the residual nitrogen by membrane separator. The nitrogen is equipped with an adsorption agent selectively adsorbed in adsorption tank from methane as receive membrane separation process gas from the first chamber of membrane separator and adsorption agent adsorbs residual nitrogen in membrane separation process gas. A pressure-swing-adsorption apparatus is provided to produce the product gas with which methane concentration is raised from membrane separation process gas, and the waste gas which nitrogen concentration is raised including the nitrogen removed after adsorption, and containing methane. The remainder is discharged. The primary membrane separation waste gas to which nitrogen concentration is raised by nitrogen supplying source gas to the first membrane separator which has separation membrane selectively transmitted, and transmitting separation membrane from methane. The primary membrane separation process gas containing the residual nitrogen in which methane concentration is raised from source gas is produced with first membrane separator. The secondary membrane separation process gas is made to merge primary membrane separation process gas. The secondary membrane separation waste gas is discharged.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the nitrogen separation apparatus.    Membrane separator (1)    First chamber (1A)    Pressure-swing-adsorption apparatus (2)    Membrane separation process gas (13)    Membrane separation waste gas (14)