To obtain an antistatic agent excellent in compatibility with a base material such as a thermoplastic resin and capable of developing its antistatic effect for a long-term even when used in a small amount, which agent comprises an ester of a copolymer of an α-olefin of a specified number of carbon atoms with maleic anhydride with an alcohol/alkylene oxide adduct.

The copolymer used is one of a 18-52C α-olefin with maleic anhydride and has a weight-average molecular weight of, desirably, 1,500-30,000, more desirably, 5,000-15,000. The α-olefin/maleic anhydride ratio of the copolymer is, desirably, 98/2 to 72/28, more desirably, 95/5 to 82/18 by weight. The alkylene oxide used is, desirably, a 2-4C one, especially desirably, ethylene oxide or propylene oxide. The ratio, total number of carbon atoms/total number of oxygen atoms, in the alkylene oxide adduct is desirably 2/1 to 4/1, particularly desirably, 2.1/1 to 2.9/1.