PURPOSE: To manufacture the magnetic iron oxide contg. cobalt which is excellent as a recording element of a magnetic recording medium by depositing a metallic compd. contg. Co on the surface of a magnetic iron oxide granule contg. ferrous iron, and then treating with steam.

CONSTITUTION: The granules of magnetic iron oxide such as γ-Fe2O3 are dispersed in an aq. liquid medium, and 0.005W10% basing on the magnetic iron oxide of a reducing agent such as hydrazine and hydrazine hydrochloride are added to reduce the iron oxide into a magnetic iron oxide contg. ferrous iron which is filtered and washed with water to obtain a cake. The cake is again dispersed in the aq. liquid medium, and a cobalt salt such as cobalt sulfate and cobalt chloride, a ferrous salt such as ferrous chloride and ferrous sulfate, and an alkali such as NaOH are added to deposit a metallic compd. contg. cobalt on the surface of the magnetic iron oxide granule. A cake obtained by filtering said material is treated with steam at 60W250°C, and the magnetic iron oxide contg. Co having an excellent magnetic characteristic such as coercive force, etc. is manufactured.