A wall is formed by adding a predetermined amount of binding agent and water to volcanic ash sediment, which is comprised of mineral silicate including at least silicic acid and aluminum oxide, kneading them, and applying to an arbitrary plane. Through a natural reaction of positive ions included within mineral silicate and moisture included within the atmospheric air, thereby exchanging ions, minus ions are generated on an application plane. The amount of minus ions generated is, in a unit volume, between three and six times as many as that of the minus ions generally included in the atmospheric air. The combination of this application plane and humidity control apparatus is used as a minus ion generation system through adjustment of amount of moisture included within the atmospheric air so as to control the ion exchanging rate. In the case where said volcanic ash sediment includes titanium oxide, it reacts with hydrogen or oxygen generating active oxygen species or oxygen free radical. Using this function of generating the active oxygen species, the application plane is utilized as an air filter.