An Ni--Ti--Nb based multiple phase alloy consists of a combined phase which comprises a phase for the hydrogen permeability and a phase for the resistance to hydrogen embrittlement. The alloy has a composition satisfying the formula: Ni.sub.xTi.sub.yNb.sub.(100-x-y) in which x=25.about.45 mol % and y=25.about.55 mol %. A metal membrane for hydrogen separation-purification is prepared using the alloy material. The alloy material is prepared by blending Ni, Ti and Nb and melting the blend. The metal membrane permits the hydrogen separation-purification and thus the resulting purified hydrogen gas can be used as a fuel and can be applied to fields of production of semiconductors.