The present invention provides a medicine for preventing or treating a diabetes, which includes 2,2-dichloro-12-(4-chlorophenyl)-dodecanoic acid or a pharmacologically acceptable salt thereof, and metformin as active ingredients. The present invention provides a method for preventing or treating a diabetes, which includes administering 2,2-dichloro-12-(4-chlo- rophenyl)-dodecanoic acid or a pharmacologically acceptable salt thereof, and metformin to a patient suffering from or having a possibility of suffering from diabetes.