In the ozone water treatment process, the silicon wafer is treated with the first ultra-pure water that includes ozone. The first ultra-pure water is refined by the ultraviolet ray sterilization method. The first ultra-pure water includes total organic carbon content of more than 1 .mu.g/liter and not more than 20 .mu.g/liter, so that the silicon wafer of the predetermined degree of cleanliness is obtained. The silicon wafer is treated by using the second ultra-pure water that has a lower TOC value than the first ultra-pure water in the ultra-pure water rinsing process (including the chemical solution cleaning process as required). The second ultra-pure water is refined by the ultraviolet ray oxidization method, and includes total organic carbon content with a concentration of 1 .mu.g/liter or less. Thus the silicon wafer of the predetermined degree of cleanliness is obtained.