The present invention provides a sugar chain synthesizer capable of continuously reacting sugar chains when a plurality of sugar chains are successively reacted. The sugar chain synthesizer of the present invention includes a plurality of vessels containing respective sugar nucleotide solutions, a plurality of vessels containing respective glycosyltransferases, and a reactor containing a primer that is a water-soluble polymer, into which the above described sugar nucleotide solution and glycosyltransferase are introduced. In the present invention, components in a reaction solution obtained in the reactor are separated through an ultrafiltration column, and a reaction product is then returned to the above described reactor, so as to continuously synthesize sugar chains. Although it is a complicated synthesis of sugar chains, it becomes possible to carry out such synthesis continuously and automatically.