PURPOSE: To manufacture the supermagnetostriction alloy including less impurities at a low cost with excellent productivity, at the time of manufacturing a rare earth-iron series magnetostriction alloy, by adopting a unidirectional solidification method with a plasma arc melting apparatus having a bottomless water-cooled copper mold.

CONSTITUTION: At the time of manufacturing a rare earth-iron series supermagnetostriction alloy having a compsn., e.g., of Tb0.3 Dy0.7 Fe1.95, a plasma torch and a bottomless cylindrical water-cooled copper mold are used. A mixed powder material of, e.g., -20 to +100 mesh constituted of a Tb-Fe master alloy, a Dy-Fe master alloy, electrolytic iron or the like is charged to a palsma arc generated from the tip of the plasma torch and is melted by the plasma arc. The molten metal of the rare earth-iron series supermagnetostriction alloy having the above compsn. solidified and heaped on a bottom stand in the water- cooled copper mold is unidirectionally solidified onto the rare earth-iron series supermagnetostriction material having the above compsn., which is gradually drawn out from the water-cooled copper mold to manufacture the rare earth-iron series supermagnetostriction alloy at low cost.