PURPOSE: To provide a new cyclic isomaltooligosaccharide useful e.g. as a clathrating agent for pharmaceuticals, foods, etc.

CONSTITUTION: Cycloisomaltoheptaose having cyclic structure formed by bonding 7 glucose molecules through α-1, 6 bonds; cycloisomaltooctaose having cyclic structure formed by bonding 8 glucose molecules through α-1, 6 bonds; and cycloisomaltononaose having cyclic structure formed by bonding 9 glucose molecules through α-1, 6 bonds. The cyclic isomaltooligosaccharide can be produced by culturing a microbial strain which belongs to the genus Bacillus and capable of producing a cyclic oligosaccharide from dextran [e.g. Bacillus sp. T-3040 strain (FERM BP-4132)] in a medium containing α-1, 6 glucan and collecting the saccharide from the cultured product.