To treat an organotin compound in a liquid at low cost. Addition of iron oxide into a liquid containing an organotin compound allows trapping of the organotin compound in the liquid by the iron oxide, and reduction of the organotin compound concentration in the liquid. Various iron oxides such as FeO, Fe.sub.2O.sub.3(iron sesquioxide), and Fe.sub.3O.sub.4 (iron oxide black) may be used, but iron oxide black is preferred. Such the method of treating a liquid may be applied to polluted seawater at a harbor to allow reduction of the organotin compound concentration in the seawater and purification of the harbor.