There are provided novel pyrimidine derivatives which has been fused with an aromatic heterocycle selected from thiophene, thiazole and pyridine or pharmaceutically acceptable salts thereof; and a pharmaceutical composition comprising said compound as an active ingredient. These compounds exhibit excellent promoting activity on insulin secretion and activity against hyperglycemia. Hence, the pharmaceutical compositions comprising such compounds as active ingredients, based on these actions, are useful for treating and/or preventing insulin-dependent diabetes (type 1 diabetes), non-insulin-dependent diabetes (type 2 diabetes), insulin-resistant diseases, obesity, and the like.