The present invention reduces the adhesiveness of trace metals in ultrapure water for cleansing products, and inhibits the metal contamination of an object to be cleansed. Provided is a metal-contamination preventive agent that contains a polymer having an ion-exchange group, such as a polystyrene sulfonate having a molecular weight of at least 100,000. Trace metals in ultrapure water are adsorbed through an ion-exchange reaction of the polymer having the ion-exchange group, and can thus be inhibited from adhering to products. Also provided is a product-cleansing method for cleansing products with ultrapure water to which said metal-contamination preventive agent has been added. Preferably, the ultrapure water to which the metal-contamination preventive agent has been added flows through a separation membrane module, and the products are cleansed with the membrane-permeated water.
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