The present invention provides novel carboxylic acid derivatives useful as an insulin sensitizer, a salt thereof or a hydrate of them, and a medicament comprising the derivative as the active ingredient. Specifically, it provides a carboxylic acid derivative represented by the following formula: (wherein L represents a single bond, or a C1 to C6 alkylene group, a C2 to C6 alkenylene group or a C2 to C6 alkynylene group, each of which may have one or more substituent groups; M represents a single bond, or a C1 to C6 alkylene group, a C2 to C6 alkenylene group or a C2 to C6 alkynylene group, each of which may have one or more substituent groups; T represents a single bond, or a C1 to C3 alkylene group, a C2 to C3 alkenylene group or a C2 to C3 alkynylene group, each of which may have one or more substituent groups; W represents a carboxyl group; X represents a single bond, an oxygen atom, or a group represented by the various substituent groups including -NR<X1>CQ<1>O- (wherein Q<1 >represents an oxygen atom or a sulfur atom; R<X1 >represents a hydrogen atom, a cyano group, a formyl group, or various groups including a C1 to C6 alkyl group and a C1 to C6 hydroxyalkyl group, each of which may have one or more substituent groups), ONR<X1>CQ<1>-, -NR<X1>CQ<1>-, -CQ<1>NR<X1>-, -NR<X1a>CQ<1>NR<X1b>-, -Q<2>SO2- and -SO2Q<2>-; Y represents a 5 to 14-membered aromatic group which may have one or more substituent groups and one or more hetero atoms, or a C3 to C7 alicyclic hydrocarbon group; and the rings Z and U may be the same as or different from each other and each represents a 5 to 14-membered aromatic group which may have 1 to 4 substituent groups and one or more hetero atoms, and the ring may be partially saturated.), a salt thereof, an ester thereof or a hydrate of them.