There is stably provided a hydrophilic cured product, such as a single-layer film, in which anionic hydrophilic groups are concentrated at a surface that is in contact with the air and which is excellent in transparency and adhesion to a substrate and tends to be rarely cracked. The hydrophilic cured product or single-layer film comprising a resin composition is produced by applying a mixture containing a polyvalent monomer (II) having two or more (meth) acryloyl groups and a compound (IV) having a specific group and a sulfonic acid group to a substrate or the like, drying the mixture if necessary, and then performing polymerization.