A novel cyclic carboxylic acid formed by the addition reaction of an unsaturated carboxylic acid with a conjugated diene compound and a metal salt thereof are disclosed. A compounding agent (A) for an antifouling paint comprising one or more substances selected from the novel cyclic carboxylic acid, a derivative of the cyclic carboxylic acid (except a metal salt), a metal salt of the cyclic carboxylic acid and a metal salt of a derivative of the cyclic carboxylic acid, and an antifouling paint composition comprising the compounding agent (A) and a copolymer (B) for a self-polishing type antifouling paint are also disclosed. The antifouling paint composition can form an antifouling coating film which is a small burden to the environment, is uniformly eroded at a given rate, is capable of maintaining excellent antifouling performance for a long period of time and is applicable to ships or the like used in the highly fouling sea area.