[Problem] To provide a pharmaceutical, particularly a compound which can be used as an insulin secretion promoter or a preventive or therapeutic agent for diabetes mellitus and the like diseases in which GPR40 is concerned. [Means for resolution] It was found that novel carboxylic acid derivatives or salts thereof, characterized in that carboxylic acid is linked to a 6-membered monocyclic aromatic ring via two atoms and said aromatic ring is linked to a nitrogen-containing bicyclic ring via a linker, have excellent GPR40 receptor agonist action. In addition, since the carboxylic acid derivatives of the invention showed excellent insulin secretion promoting action and blood glucose reducing action, they are useful as an insulin secretion promoter and a preventive or therapeutic agent for diabetes mellitus.