An Nb--Ti--Co alloy having both good hydrogen permeability and good hydrogen embrittlement resistance comprises one of Fe, Cu or Mn as a fourth element, incorporating from 1 to 14 mol %. The content of Mn, if any, is preferably from 1 to 9 mol %. The desired hydrogen permeability can be attained by the (Nb, Ti) phase and the desired hydrogen embrittlement resistance can be attained by the CoTi phase, making is possible to obtain excellent hydrogen permeability and excellent hydrogen embrittlement resistance. None of Fe, Cu or Mn can impair these properties. Fe, Cu or Mn can replace some of the Co elements. Fe, Cu or Mn enhances the workability of the alloy.