By suppressing oxidation of sulfide minerals in sulfide ore due to bacteria or the like, this invention prevents the elution of heavy metals from the sulfide ore, and reduces the decrease in flotation performance when processing sulfide ore that is stored in a stockpile. Also, the invention makes it easier to process acidic wastewater from a stockpile or tailings dumpsite. In order to accomplish this, oxidation of sulfide minerals in sulfide ore is suppressed by adding an antioxidant, which contains plant polyphenol and whose main component is an organic acid that contains a carboxyl group, to the sulfide ore stored in a stockpile or tailings dumpsite.