The present invention provides a transglycosylation method and a protein catalyzing a transglycosylation reaction. A reaction of transferring a galactosyl.beta.1.fwdarw.3N-acetylgalactosaminyl group from a sugar donor containing a sugar chain having a galactosyl.beta.1.fwdarw.3N-acetylgalactosaminyl group bound thereto at the .alpha.-position, to a sugar chain acceptor, is carried out in the presence of a protein having an endo-.alpha.-N-acetylgalactosaminidase activity and a transglycosylation activity.