ABSTRACT A packaged product formed by enclosing a content material, such as food or beverage, sanitary products or medical products, within a packaging material including at least a layer of hydrophilic resin, is heat-treated with hot water for, e.g., boil sterilization or retort sterilization. The hot water is caused to contain a water-soluble compound in an amount substantially exceeding a level contained in tap water, whereby it becomes possible to suppress the opalescence of the packaging material causing inferior appearance or transparency and leading to a lowering in gas-barrier property, which has been problematic in the conventional boil or retort hot water treatment.