PURPOSE: To obtain an antifouling agent capable of strongly inhibiting the damage by fouling organisms for a long period without elution or persistence of toxic substances such as a heavy metal and adversely affecting ecosystems.

CONSTITUTION: This antifouling agent composition is obtained by blending (C) a compound selected from fluorodichloromethyl-thiophthalimide, dimethylphenylfluorodichloromethylthiosulfamide and dichlorophenyl-dimethylurea in an antifouling composition comprising (A) a phenolic compound selected from the group consisting of 4-21C alkylphenolic compounds and aromatic- substituted phenolic compounds of formula I [X is CO, NH, CH2-O, NH-CO, COO, CH2-O-(C=O), CH2, C(CH3)2 or formula II; (m) is 0 or 1; Y is H or OH; (n) is 1-5], e.g. 2,4-dihydroxy-benzophenone as an active ingredient and further (B) a water-repellent compound selected from silicone oils, petroleum waxes and liquid olefins blended therewith. The amount of the ingredient (C) is 0.5-20wt.%, preferably 1-12wt.% based on the total amount of the ingredients (A) and (C) or the ingredients (A), (B) and (C).