The present invention relates to a method for imparting hydrophilicity to a substrate whereby high hydrophilic properties and water-holding properties can be maintained for a long period of time. According to the present invention, an SiO2 film is formed directly or through an undercoat layer on a substrate under a reduced pressure of 100 Pa or less and immediately after the SiO2 film is formed, the SiO2 film is treated with water. Before forming the SiO2 film, it is also desirable that an undercoat layer consisting of a TiO2 film, Al2O3 film, Nb2O5 film, a laminated film prepared by laminating the TiO2 film on the Al2O3 film, a laminated film prepared by laminating the TiO2 film on the Nb2O5 film, or a low emissivity film be formed on a substrate and the SiO2 film be then formed on the undercoat film to serve as an SiO2 composite film.