PURPOSE: To prevent the generating of rust by diffusing carbon on the surface of an R-B-Fe magnet.

CONSTITUTION: An R-B-Fe magnet is heated and held in atmosphere which contains CO gas and small quantity of CH4 gas or C3H6 gas and further anhydrous NH3 gas as required, heated and held in atmosphere which contains NH3 gas by a method of carburizing the surface of the magnet or at 450W690°C, and by a method of nitriding the surface of the magnet. The formation of a nitrided layer by the nitriding causes a nitride with these elements, and is effective at the point of preventing the generation of rust when containing Al, Cr, Ti, V and/or Mo.