NOVELTY - The method comprises: preparing a porous support; preparing a palladium (Pd)-containing layer comprised of Pd-containing particles on a first surface of the porous support directly or on a porous shielding layer located on the first surface of the porous support; polishing the Pd-containing layer; introducing a sealing layer that blocks the absorption of an electroless plating solution on a second surface of the porous support; and forming a Pd-containing dense membrane from the Pd-containing particles of the Pd-containing layer.    USE - The method is useful for preparing a hydrogen separation membrane, which is useful in a method for preparing hydrogen using a device (all claimed), where the hydrogen is useful as a raw material of chemical synthesis, as reducing gas in a semiconductor manufacturing process and as a fuel of fuel cells after being separated from other reformates or reactant gas.    ADVANTAGE - The method is capable of preventing the plating of Pd inside a porous support and a porous shielding layer. The compact membrane allows separation of hydrogen in an eco-friendly manner, can be simply operated, and has improved selective permeability and performance.    DETAILED DESCRIPTION - The method comprises: preparing a porous support; preparing a palladium (Pd)-containing layer comprised of Pd-containing particles on a first surface of the porous support directly or on a porous shielding layer located on the first surface of the porous support; polishing the Pd-containing layer; introducing a sealing layer that blocks the absorption of an electroless plating solution on a second surface of the porous support; forming a Pd-containing dense membrane from the Pd-containing particles of the Pd-containing layer by immersing the porous support provided with the Pd-containing layer on the first surface and the sealing layer on the second surface in a Pd-containing electroless plating solution; and removing the sealing layer from the porous support. The electroless plating solution penetrates toward the first surface of the porous support. The Pd-containing dense membrane grows toward the first surface of the porous support from the Pd-containing particles of the Pd-containing layer. The Pd is not plated on the porous support and the porous shielding layer by the Pd-containing dense membrane growing toward the first surface of the porous support from the Pd-containing particles of the Pd-containing layer. The first surface of the porous support is filled with particles having sizes smaller than gaps or micropores. The Pd-containing layer comprised of the Pd-containing particles is prepared using sputtering process. The Pd-containing particles are made of Pd or its alloy. The sealing layer is formed by filling pores or gaps of the second surface of the porous support using a material present as a solid at a temperature of the electroless plating and present as a liquid at temperatures higher than the temperature of the electroless plating. INDEPENDENT CLAIMS are included for:    (1) a hydrogen separation membrane;    (2) a device for preparing hydrogen; and    (3) a device for preparing a hydrogen separation membrane.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a hydrogen separation membrane.    Housing (10)    Shielding unit (20)    Elastic body (40)    Hydrogen separation membrane. (100)