An object of the present invention is to provide a composition for forming an environment friendly antifouling coating film that is unlikely to cause a hairline crack and like coating film defects even when immersed in seawater for a long time, and that prevents or inhibits attachment of slime. An antifouling coating composition of the invention comprises: (A) a triorganosilyl ester-containing copolymer obtained by a mixture of (a) a triorganosilyl (meth)acrylate monomer represented by a general formula (1): R1 is a hydrogen atom or a methyl, and R2, R3, and R4 are equal or different each other, and each represent an alkyl group having 3 to 6 carbons and branched on α-position or a phenyl group, and (b) an ethylenically unsaturated monomer copolymerizable with the triorganosilyl (meth)acrylate monomer, (B) a modified rosin salt which is a salt of a modified rosin and zinc and/or copper, the modified rosin containing, in an amount of 80 weight % or more, two or more members selected from the group consisting of dihydroabietic acid, tetrahydroabietic acid, dehydroabietic acid, pimaric acid, isopimaric acid, and dihydropimaric acid.