To provide an antifouling agent for an aqueous coating material, which is added to the aqueous coating material, is excellent in stability after added, and prevents a surface coated with the aqueous coating material from rain streaks or blackishness, maintaining the luster of the coated surface.

This antifouling agent contains a cross-linked polymer formed by the ring-opening addition polymerization of an epoxy compound selected from the group consisting of monool glycidyl ethers and polyol glycidyl ethers with a hydroxylated compound selected from the group consisting of monools, polyols, and polyoxyalkylene compound formed by substituting a monool or a polyol with a polyoxyalkylene group.