A coating composition which develops no coating film defects, such as peeling, in water with time, wears away at a constant rate, has excellent long-term antifouling properties, is applicable even to aluminum boats, and is highly safe environmentally; an antifouling coating film; an underwater structure with unsusceptibility to fouling; and an antifouling method. The coating composition is characterized by containing a copolymer comprising both of an ingredient A) represented by the following formula (1) and an ingredient B) derived from a polymerizable unsaturated metal salt compound represented by R5-(CH2)k-COO-M-Lq.
