Carboxylic acid compound or salt or hydrate thereof useful as insulin sensitizer, represented by the formula ##STR00001## wherein R.sup.1 represents hydrogen, hydroxyl, halogen, carboxyl, or C.sub.1-6 alkyl, L represents single bond or C.sub.1-6-alkylene, C.sub.2-6-alkenylene, or C.sub.2-6-alkynylene, M represents single bond or C.sub.1-6-alkylene, C.sub.2-6-alkenylene, or C.sub.2-6-alkynylene, T represents single bond, or C.sub.1-3-alkylene, C.sub.2-3-alkenylene, or C.sub.2-3-alkynylene, W represents carboxyl, ##STR00002## represents single bond, X represents single bond, oxygen, --NR.sup.X1CQ.sup.1O--, --OCQ.sup.1NR.sup.X1--, --CQ.sup.1NR.sup.X1O--, ONR.sup.X1CQ.sup.1--, --Q.sup.2SO.sub.2--, or --SO.sub.2Q.sup.2--, Y represents 5- to 14-membered aromatic group, and ring Z represents 5- to 14-membered aromatic group.