The present invention provides a compound of the formula: wherein ring A is an optionally substituted 5 to 10-membered aromatic ring; R¹ and R² are the same or different and each is an optionally substituted hydrocarbon group or an optionally substituted heterocyclic group; X is a bond and the like; and L is a divalent hydrocarbon group, and a salt thereof, except 3-(aminomethyl)-2,6,7-trimethyl-4-phenyl-1(2H)-isoquinolinone, 3-(aminomethyl)-2-methyl-4-phenyl-1(2H)-isoquinolinone, 3-(aminomethyl)-6-chloro-2-methyl-4-phenyl-1(2H)-isoquinolinone and 3-(aminomethyl)-2-isopropyl-4-phenyl-1(2H)-isoquinolinone. The compound shows a superior peptidase-inhibitory activity and is useful as an agent for the prophylaxis or treatment of diabetes and the like. ##STR1##