PURPOSE: To remove impurities from gaseous methane (CH4) without discharging hazardous substances, without the danger of the purified gas later to be polluted again and yet without decomposition of CH4 by using a getter alloy which is not hydrogenated.

CONSTITUTION: CH4 containing H2O and O2 or the like is circulated through a purification device 10 having a room 14, equipped with an impure gas inlet 12 and a purified gas outlet 18 and storing pellets of Zr-V-Fe alloy 16 with a composition by weight of 45-75% Zr, 20-50% V and 5-35% Fe. Zr-V-Fe alloy 16 has previously been activated at 400-950°C to be gas adsorptive. The impure CH4 is brought into contact with the alloy at <400°C at which temp. CH4 is not adsorbed. This is useful as the methane for the formation of a thin film of diamond or the like.