A coral is washed with purified water or tap water, and a crushed coral is heat-treated. Proteins and other organic matter are incorporated between calcium molecules in the coral skeleton, and the organic matter is decomposed and released as volatile components. In the skeleton from which organic matter is released, a calcium skeleton having a porous structure is formed, and the surface area of pores is increased. A hydrogen gas can be retained in the structure by adsorbing a hydrogen gas to the pores in such a calcium skeleton a porous structure (micropores). The resulting coral powder, with which ions dissolved in water are easily extracted, shows a low oxidation reduction potential when dissolved in water.