The cationic ring-opening reaction of 1,6-anhydro-2,3,4-tri-0-benzyl-beta-D-mannopyranose was applied to the synthesis of alpha-(1-->6)-linked manno-oligosaccharide which is a part of natural, high-mannosetype oligosaccharides. Dimethoxymethane was added into the reaction system as a transfer reagent in order to diminish the molecular weight of the products. It was confirmed by gel permeation chromatography measurements that additional dimethoxymethane distinctly lowered the molecular weight of the oligomerization products. The presence of the -OCH3 group, which was transferred from dimethoxymethane, at the reducing terminal of the obtained oligosaccharide derivatives was confirmed by C-13 NMR measurements. In addition, methyl 2,3,4-tri-0-benzyl-6-O-methoxymethyl-alpha-D-manopyranoside was synthesized as a model compound. From the comparison of the C-13 NMR spectrum of the reaction product with that of the model compound, -O-CH2-O-CH3 group binding to the nonreducing end was found to be present. These results showed the mechanism of the transfer reaction. Since another product was detected by using C-13 NMR spectroscopy, isolation of the products was carried out by silica gel column chromatography. It was shown by H-1 and C-13 NMR spectroscopies, COSY, HMQC, and HMBC that a 4,6-methylidene structure was also formed at the nonreducing end of the oligosaccharide derivatives.