Cyclic onium compounds reresented by the following structural formula (I), glucosidase inhibitors using such compounds, and antidiabetic drugs or food containing such glucosidase inhibitor. wherein A.sup.- is an aniom; m is an interger between 1 and 6, n is 0 or 1, X.sup.+ is S.sup.+ or N.sup.+Q (where Q is H or an alkyl of 1 to 4 carbon atoms).