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.