As the density of the large-scale integrated (LSI) chip increases, less contamination of the external surface of the wafers is permitted. At the same time, it is necessary to maintain the uniformity of surfaces of a wafer and also uniformity among wafers. To meet these requirements, the environmental purification of film-forming processes and etching processes is indispensible, and perfect wet cleaning of wafer surfaces is also important. This is the ultimate purpose of a stable supply of ultrapure water of the lowest impurity concentration.