An aqueous slurry of a magnetic iron oxide powder is mixed with a suspension prepared by neutralizing an aqueous ferrous salt solution with an alkali to form ferrous hydroxide and then adding an aqueous cobalt salt solution thereto, and the excess OH concentration of the mixed slurry is adjusted to 0.05 to 3 mol/l, thereby obtaining a cobalt-and ferrous iron-containing ferromagnetic iron oxide. The cobalt-and ferrous iron-containing ferromagnetic iron oxide is excellent in coercivity, distribution of coercivity and other magnetic properties, and the magnetic tape produced by using such iron oxide is also excellent in coercivity and switching field distribution.