Provided is an antifouling coating composition giving coating films which show satisfactory dissolution properties and a satisfactory antifouling effect in seawater from the beginning and stably retain the dissolution properties and antifouling effect for a long period and which neither peel off nor crack in seawater for a long period and have satisfactory physical properties. The present invention provides a copolymer for the antifouling coating composition, the copolymer being obtained by copolymerizing a mixture of a triorganosilyl methacrylate monomer (a) represented by general formula (1) and one or more polymerizable unsaturated monomers (b) other than the monomer (a), the monomers (b) comprising methyl acrylate, the monomer (a) accounting for 25-65 mass% of the total mass of the monomer (a) and the monomers (b), and the methyl acrylate accounting for 25-75 mass% of the total mass of the monomer (a) and the monomers (b). (In formula (1), R1 is a methyl group and R2, R3, and R4 may be the same or different and are each selected from among C3-8 branched alkyl groups and a phenyl group.)