A copolymer having a weight-average molecular weight of 1,000 to 100,000 which comprises, as repeating structure units, both of a succinimide unit represented by the structural formula (1) and a hydroxycarboxylic acid unit represented by the structural formula (2) wherein R is a methyl group or a hydrogen atom, and a process for preparing a copolymer which comprises a polymerization step of heating a mixture of aspartic acid and a cyclic ester compound.