A dipeptidyl peptidase IV inhibitor which is satisfactory in respect of activity, stability and safety and has an excellent action as a pharmaceutical agent. A compound represented by the following general formula or a pharmaceutically acceptable salt thereof: wherein R.sup.1 and R.sup.2 each represents hydrogen, an optionally substituted C1-6 alkyl group, or --COOR.sup.5 whereupon R.sup.5 represents hydrogen or an optionally substituted C1-6 alkyl group, or R.sup.1, R.sup.2, and a carbon atom together represent a 3- to 6-membered cycloalkyl group, R.sup.3 represents hydrogen or an optionally substituted C6-10 aryl group, R.sup.4 represents a hydrogen or a cyano group, D represents --CONR.sup.6--, --CO-- or --NR.sup.6CO--, R.sup.6 represents hydrogen or an optionally substituted C1-6 alkyl group, E represents --(CH.sub.2).sub.m-- whereupon m is 1 to 3, --CH.sub.2OCH.sub.2--, or --SCH.sub.2--, n is 0 to 3, and A represents an optionally substituted bicyclic heterocyclic group or bicyclic hydrocarbon group.