Mineral particles consisting essentially of metallic magnesium particles subjected to special processing are accommodated directly in a microporous cartridge made of a sintered polypropylene material, and the cartridge is closed. The cartridge is put in a closed raw water container, whereby a large amount of hydrogen gas is generated in a short period of time and released from the whole surface of the cartridge in the form of microbubbles so as to be dissolved in raw water in the container.