PURPOSE: To improve collapsibility of bulky reaction generating material at the time of executing wet-treatment and to obtain powder having good corrosion resistance by incorporating two or more kinds of KBr, CaBr2 and CaCl2 in a mixed material subjected to heating in reduction diffusion method.

CONSTITUTION: After heating the mixed material of rare earth metal oxide powder and the other metal powder and one or more kinds of alkali metal, alkali earth metal and hidride thereof under inert gas atmosphere or under vacuum, the reaction generating mixed material is treated under wetting condition. In this method, in this mixed material subjected to the heating, further two or more kinds of KBr, CaBr2 and CaCl2 are incorporated. It is considered that these salts lower the starting temp. of melting to the temp. lower than that of the conventional alkali (earth) metal chloride and bulking acceleration of the reaction generating material with over raising of the temp. is suppressed with these salts as absorbing agent of heat generated with heating reduction. Further, at the time of executing the wet-treatment under coexistence of KBr and/or CaBr2, removal of chlorine is facilitated and the corrosion resistance can be improved.