There is provided an antistatic agent that preserves its antistatic property for a long period of time and has the appearance and the transparency that are not impaired when kneaded into a resin, and a resin composition containing the antistatic agent. An antistatic agent including a block copolymer which is obtained from a reaction product of a polyolefin (a) having one end being acid-modified and a diol and/or a diamine (b) having a polyoxyalkylene chain, and in which a part or all of acid groups remaining in the reaction product is neutralized with an alkaline substance. A thermoplastic resin composition including the antistatic resin. A film or sheet including the thermoplastic resin composition.