PURPOSE: To carry out the oxidation of a high concentration iron salt solution with a simple apparatus, by floating a proper amount of small spherical particles on the surface of a ferrous salt solution containing ferrous hydroxide precipitate, and stirring the particles together with the solution.

CONSTITUTION: For example, a plastic vessel is floated and fixed in a thermostatic bath, and a ferrous salt solution containing ferrous hydroxide precipitate is poured into the vessel. Small spherical particles having a diameter of about 1W 2mm and made of a foamed resin, etc. are put into the vessel to cover the whole surface of the solution. When the solution is stirred at 30W40°C, the ferrous salt is oxidized to obtain the objective iron oxide.