Abstract: To provide a method for producing a filtration filter that can simplify the process for providing clean water or freshwater. By etching silicon substrate 1 using masking film formed on a surface of substrate 1 and having numerous openings to expose portions of the surface, numerous circular holes 2 with an approximate diameter of 100 nm are formed in substrate 1. Diameter (D1) at minimum-diameter portions 4 near the openings of circular holes 2 to be reduced by silica film 3 is adjusted to be 1 nm˜100 nm by depositing silica film 3 on the inner surfaces of circular holes 2.

Abstract: Provided is a water treatment device with which organic substances contained in raw water to be treated are decomposed to thereby alleviate the load to be imposed on a downstream filter and with which it is possible to avoid corrosion of the piping or the like.