PURPOSE: To improve the resistance of the titled magnetic powder to oxidation and to obtain safely and easily a plastic magnet by treating the powder of a magnet consisting essentially of the intermetallic compound between a rare earth metal and iron or cobalt and having specified particle diameter under specified consitions.

CONSTITUTION: A magnet, consisting of the intermetallic compd. constituted essentially of a rare earth metal and iron or cobalt, is prepared. The powder of said magnet having 1W150μm particle diameter is exposed to an atmosphere contg. oxygen at 50W250°C. Then the powder is brought into contact with a phosphours compd. contg. acrive protons having ≤4pKa acidity constant such as phophoric acid, acidic phosphoric ester, and dialkyl dithiophosphate. The magnetic powder obtained by said method is not oxidized when the plastic magnet using said magnetic material as the starting material is manufactured. Accordingly, a plastic magnet having excellent magnetic characteristics can be obtained.