an .alpha.-selective glycosylation process of a glycosylation reaction between a sugar structure of hexose or an aldose having a chain with six or more carbon atoms in the molecuse, which is a monosaccharide or a reducing end of an oligosaccharide with two or more monosaccharide residues connected by glycosidic linkages (an oligosaccharide with two monosaccharide residues is disaccharide) (also referred to as "a disaccharide to an oligosaccharide") or a sugar chain and alcoholic hydroxyl group of thiol group, to obtain a sugar structure glycoside containing .alpha.-glycoside at a high ratio, under the ring-shaped formation of a protective group in a silyl acetal structure over hydroxyl groups at positions 4 and 6 in the sugar structure. By the process, highly selective .alpha.-glycosylation of sugar structure can be progressed in a simple and efficient manner.