An aqueous solution is prepared comprising 5 to 25% by weight of an anionic surfactant, 0.5 to 20% by weight of an amphoteric surfactant, 4 to 6% by weight of a nonionic surfactant and, if necessary, an antiseptic and the aqueous solution is adjusted with an organic acid to pH in the range of 4.5 to 7.0, which is used as an undiluted solution of a hydrophilicity maintaining and promoting agent for a polysilazane-containing coating film. The undiluted solution is diluted with water to from 3 to 15 times and the resulting solution is used as a hydrophilicity promoting agent. While the undiluted solution is diluted with water to from 30 to 70 times and the resulting solution is used as a hydrophilicity maintaining agent. The hydrophilicity promoting agent is applied on an anti-fouling coating film which is formed by applying on a substrate a coating solution comprising polysilazane and, if necessary, silica conversion catalyst, and thereby the coating film is made hydrophilic in an extremely short time. Further the hydrophilicity maintaining agent is used for removing stains which adhere to the coating film and maintaining the hydrophilicity of the film.