PURPOSE: To obtain the titled iron oxide suitable for a raw material for anticorrosive coating materials, by oxidizing a waste water containing ferrous iron ions in a high concentration with iron-oxidizing bacteria, separating a solid material from the liquid, adding an alkali to the solid material, aging the resultant mixture, and subjecting the aged mixture to hydrothermal synthesis.

CONSTITUTION: A liquid, e.g. waste water, containing ferrous ions and heavy metal ions in high concentration, and discharged from various factories as an object liquid is oxidized with iron-oxidizing bacteria cultivated in a fluid containing ferrous ions in a high concentration to form ferric precipitates, which are then separated from the liquid. An alkali agent is then added to the separated solid material, and the resultant mixture is aged under stirring. The aged mixture is then subjected to hydrothermal synthesis to give of the aimed iron oxide. Preferably, the concentration of the ferrous ions in the culture fluid is ≥30g/l, and that of the ferrous ions in the liquid to be oxidized is ≥10g/l.