PURPOSE: To suppress the deterioration in the saturation magnetization of magnetic Cr oxide powder by coating the particle surface of the magnetic Cr oxide powder by oxide or hydroxide contg. ≥1 kinds of elements of various metals such as Mg and Al.

CONSTITUTION: Coating of the particle surface of the magnetic Cr oxide powder is executed by immersing the magnetic Cr oxide powder into an aq. soln. contg. the oxide or hydroxide contg. at least ≥1 kinds of the elements selected from Mg, Al, Si, Ca, Ti, Cr, Mn, Fe, V, Co, Ni, Cu, Sn, and Zn to deposit the oxide or hydroxide contg. at least ≥1 kinds of these elements on the particle surface, then heating the powder to 100W700°C to remove the water of the surface layer and to prevent reaction of the Cr(IV) component and the water in the air. Conversion of Cr(IV) to Cr(III) or Cr(VI) by reaction with the water is thereby obviated and the deterioration in saturation magnetization with lapse of time is satisfactorily suppressed.