PURPOSE: To efficiently treat heavy metal-contg. waste water containing ferrous ion with high concentration, by oxidatively treating waste water using iron- oxidizing bacteria cultivated in a liquid containing impurities with high concentration, and reusing precipitately formed sludge.

CONSTITUTION: Fe2+ existent in heavy metal-contg. waste water containing at least Fe2+ and other metal ions with high concentration is oxidized into Fe3+ using iron-oxidizing bacteria cultivated in a conc. liquid containing impurities. Aeration may be optionally performed at an ordinary temp. Sludge containing iron-oxidizing bacteria precipitately formed by the oxidizing treatment is returned to an oxidizer tank and reused therein. Diatomite as a carrier for the bacteria and a tonic may be added to the oxidizer tank at need. By this method, the heavy metal-contg. waste water can be treated at a low cost, and the amount of neutralized precipitates formed by neutralizing treatment can be reduced.