NOVELTY - A supporting base comprises layer of multilayer structure on side of a gas separation membrane formation having a porous sintered metal made of particles to be sintered having an average particle diameter of at most8 mum, and a maximum particle diameter of at most45 mum and has a percentage of open area of at least30%.    USE - For gas separation membrane used particularly in the production of high purity hydrogen gas for us in e.g. fuel cells.    ADVANTAGE - The invented supporting base has a high gas permeability and enables emissions of operation, e.g. welding.    DETAILED DESCRIPTION - A supporting base comprises a multilayer cylinder of porous sintered metal. The layer of the multilayer structure on the side in contact with the separated gas comprises a porous sintered metal having a relative density of at least30% and is made of particles to be sintered having an average particle diameter of at least10 mum. The layer of the multilayer structure on the side of a gas separation membrane formation comprises a porous sintered metal made of particles to be sintered having an average particle diameter of at most8 mum, and a maximum particle diameter of at most45 mum and has a percentage of open area of at least30%. The multilayer cylinder of porous sintered metal has an outer diameter of at most15 mm, and a length of at least100 mm. INDEPENDENT CLAIMS are also included for:    (A) a method for producing the supporting base for gas separation membrane comprising continuously extruding raw metal powder materials corresponding to each constituent layer of the multilayer structure in the order starting from the raw metal powder material constituting the innermost layer to the raw metal powder material constituting the outermost layer such that the newly extruded layer surrounds the preceding layer to produce a green cylinder of multilayer structure; and sintering the green cylinder to produce the supporting base for gas separation membrane; and    (B) a gas separation filter produced by forming a gas separation membrane on the supporting base for gas separation membrane on the side of the gas separation membrane formation.    DESCRIPTION OF DRAWING(S) - The figure is a schematic cross-sectional view of the main part showing the production method of the supporting base.    Nozzles (2, 3)    Raw material inlets (12, 13)    Passage (14)    Outlet (15)