Effects of a gap/open space between double membranes of Ni/Ni and Pd/Ni on hydrogen permeation through the double membranes are studied. For easy detection of permeated H, T is introduced. For Ni/Ni and Pd/Ni, the influence of the gap on hydrogen permeation is not appreciable, while the permeation for Ni/Pd is significantly reduced because the gap holds H2O produced by the reaction of permeated hydrogen and the surface oxide of Ni facing toward the gap; consequently, the partial pressure of H2O in the gap becomes high and subsequent reduction of the surface oxide is prohibited. From these findings, a new double-walled tube concept for the reduction of T permeation is proposed with a combination of a rather thin front tube with its back side oxidized as a permeation barrier and a thick tube as a structure material.