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