There is provided a novel pyrrolidine derivative represented by the formula (I) or a salt thereof having an excellent DPP-IV inhibitory activity and an excellent pharmacokinetic. On the basis of the action as such, the compound of the present invention is useful for treatment and/or prevention of insulin-dependent diabetes mellitus (type 1 diabetes mellitus) and particularly for non insulin-dependent diabetes mellitus (type 2 diabetes mellitus), insulin-resistant diseases, obesity, etc. [In the formula, -A- is a single bond, --O--, --S--, --NH-- or --N(lower alkyl)-; and --B is lower alkyl, lower alkenyl, lower alkynyl, cycloalkyl, aryl, non-aromatic hetero ring or aromatic hetero ring and each of those groups may be substituted.]