The object of the present invention is to provide an .alpha.-isomaltosylglucosaccharide-forming enzyme, process of the same, cyclotetrasaccharide, and saccharide composition comprising the saccharide which are obtainable by using the enzyme; and is solved by establishing an .alpha.-isomaltosylglucosaccharide-forming enzyme which forms a saccharide, having a glucose polymerization degree of at least three and having both the .alpha.-1,6 glucosidic linkage as a linkage at the non-reducing end and the .alpha.-1,4 glucosidic linkage other than the linkage at the non-reducing end, by catalyzing the .alpha.-glucosyl-transfer from a saccharide having a glucose polymerization degree of at least two and having the .alpha.-1,4 glucosidic linkage as a linkage at the non-reducing end without substantially increasing the reducing power; .alpha.-isomaltosyl-transferring method using the enzyme; method for forming .alpha.-isomaltosylglucosaccharide; process for producing a cyclotetrasaccharide having the structure of cyclo{66)-.alpha.-D-glucopyranosyl-(163)-.alpha.-D-glucopyranosyl-(166)-.- alpha.-D-glucopyranosyl-(163)-.alpha.-D-glucopyranosyl-(16} using both the .alpha.-isomaltosylglucosaccharide-forming enzyme and the .alpha.-isomaltosyl-transferring enzyme; and the uses of the saccharides obtainable therewith.