To provide a method for manufacturing such a diplophase alloy for separating/refining hydrogen as to have high rolling-formability and a high hydrogen-permeation coefficient.

This diplophase alloy is a Nb-Ti-Ni alloy formed of a diplophase comprising a phase having hydrogen permeability and a phase having hydrogen embrittlement resistance. The method for manufacturing the diplophase alloy includes heat-treating the Nb-Ti-Ni alloy at a temperature higher than 1,000°C for 100 hours or longer. The manufacturing method also includes plastic-working the Nb-Ti-Ni alloy and subsequently heat-treating it at a temperature higher than 1,000°C. Thus manufactured diplophase alloy can obtain the hydrogen permeation coefficient almost equal to that of the as-cast alloy.