To provide a hydrophilizing agent for a water-based coating exhibiting enhanced floating functions by using a thermally responsive polymer compound making a sharp switch of phase transition from hydrophobic to hydrophilic around a phase transition temperature.

The hydrophilizing agent comprises a thermally responsive polymer compound carried on a silica particle, where the polymer compound is obtained by copolymerizing a hydrophobic monomer, which exhibits hydrophobicity when polymerized, and has a phase transition temperature where it switches reversibly between hydrophilicity and hydrophobicity. Thus, the hydrophilizing agent gives sharp phase transition from hydrophilic to hydrophobic to easily float up to the surface in a coating.