PROBLEM TO BE SOLVED: To provide a novel superior antitumor substance capable of being easily prepared from natural supply sources.SOLUTION: A composition including an oyster hot-water extract, specifically an oyster hot-water extract obtained under pressurization at 125°C, is provided. The oyster extract is different in molecular weight and physical properties of an active ingredient in comparison with a conventional extract. The composition exhibits such superior effects that NK cells are activated and melanoma can be treated.