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 (I): ##STR00001## (wherein Y, L, X, T, Z, U, M and W are defined in the specification) and a salt thereof, and ester thereof or a hydrate of them.