The present invention realized the production of a water product having excellent physiological activity such as health promoting activity and the utilization of the water, in which ultrafine gold particles having a diameter as small as 1-2 figures in micron order, much smaller than any ordinary fine gold particles, are dissolved.According to the present invention, an aqueous ultrafine gold particle solution is produced using an apparatus comprising a pressure-resistant vessel equipped with a high-pressure water tank, a jet nozzle, an ignition device, and a combustion chamber, in which a gas mixture comprising hydrogen and oxygen is combusted in highly pressurized water, in which gold leaf fragments are suspended, and then the gold leaf fragments are heated and melted by the resulting combustion gas.