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 adsorbedthrough 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 productswith 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 aseparation membrane module, and the products are cleansed with the membrane-permeated water.