After various sugar nucleotide solutions and glycosyltransferases (or primers) have been mixed, they are introduced into a reaction tank or column (5) with primers or glycosyltransferases immobilized thereon. Then solutions coming out of the reaction tank (5) are led to an ultrafiltration column (6). The synthesizer is equipped with a flow path for ensuring that glycosyltransferases or primers separated by the ultrafiltration column (6) are returned into a container for storing each solution in a sample injector (3).