A pharmaceutical agent having GPR40 receptor agonistic action, particularly a compound which is useful as an insulin secretagogue or an agent for preventing and/or treating diabetes mellitus. The present inventors have examined a compound having GPR40 receptor agonistic action, confirmed that an oxadiazolidinedione compound which has a substituent such as a benzyl group, etc. linked with a substituent such as a phenyl group, etc. through a linker at the 2-position of an oxadiazolidinedione ring, or a pharmaceutically acceptable salt thereof has an excellent GPR40 agonistic activity, and thus completed the invention. The oxadiazolidinedione compound has excellent insulin secretagogue action and anti-hyperglycemic action, and therefore can be used as an insulin secretagogue or an agent for preventing and/or treating diabetes mellitus.