To obtain a coating material composition, to prepare an antifouling coating film, to provide an underwater structure and a method for forming the antifouling coating film on the underwater structure that exhibits excellent marine organism-antifouling effect for a long period of time without film peeling off or cracking with the passage of time to keep the environment safe, since the coating film is consumed slowly at a certain speed after immersed into sea water.

This coating material composition contains at least one kind selected from a specific organic silyl hydrolytic resin and a specific diphenyl borane.