A polyester plasticizer which is obtained by reacting a diol component (a) consisting of 100 parts by mole of 2-methyl-1,3-propanediol, 10 to 1000 parts by mole of 3-methyl-1,5-pentanediol and 0 to 1000 parts by mole of other aliphatic diol with an organic dicarboxylic acid components (b) consisting of 100 parts by mole of adipic acid and 0 to 100 parts by mole of other organic dicarboxylic acid and a chain terminator (c) consisting of a monovalent aliphatic alcohol or a monovalent aliphatic organic acid and which has an average molecular weight of 500 to 5000. This plasticizer is excellent in plasticizing efficiency and can impart excellent oil resistance to synthetic resins such as chlorine-containing resins.