A sugar donor having a glucuronic acid or iduronic acid derivative at the reducing end and in which a leaving group is added to the reducing end hydroxyl group to be glycosylated and the other hydroxyl groups and the carboxyl groups are protected, is subjected to glycosylation reaction with a sugar acceptor having a N-acylgalactosamine derivative at the reducing end and in which the non-reducing end hydroxyl group to be glycosylated is free and the other hydroxyl groups are protected, in the presence of (A-3) a particular promoter. Provided be can a process for preparing an oligoglycosaminoglycon of an intended chain length composed of four or more constituent sugars in highly stereoselectively, high yield and high purity.