The object of the present invention is to provide an .alpha.-isomaltosyl-transferring enzyme which forms a cyclotetrasaccharide having the structure of cyclo{.fwdarw.6)-.alpha.-D-glucopyranosyl-(1.fwdarw.3)-.alpha.-D-glucopyr- anosyl-(1.fwdarw.6)-.alpha.-D-glucopyranosyl-(1.fwdarw.3)-.alpha.-D-glucop- yranosyl-(1.fwdarw.} from 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; microorganisms which produce the enzyme; process for producing the enzyme; cyclotetrasaccharide or saccharide compositions comprising the same; and uses thereof.