By using an apparatus composed of a high-voltage discharge generator having a high-voltage large-current discharge power supply, a noble metal electrode, and a counter electrode, a water tank, and a container having a water supply port for supplying water to the water tank and an outlet port for discharging ultrafine dispersion water of noble metal ultrafine particles, plasma discharge is caused between the noble metal electrode and the counter electrode in the water to bring the noble metal ion vapor into contact with water and to cause ultrafine dispersion. Thus, an ultrafine dispersion water of ultrafine particles containing a noble metal the concentration of which can be adjusted can be efficiently produced by means of a small producing apparatus with high safety. The produced water is useful as health water.