A 2-iminoimidazole derivative represented by the formula: <;CHEM>; äwherein R<;1>;, R<;2>; and R<;3>; represent hydrogen, optionally substituted C1-6 alkyl, etc.; R<;6>; represents hydrogen, C1-6 alkyl, C1-6 alkyloxycarbonyl, etc.; Y<;1>; represents a single bond, -CH2-, etc.; Y<;2>; represents a single bond, -CO-, etc.; and Ar represents hydrogen, a group represented by the formula: <;CHEM>; Äwherein R<;10>;-R<;14>; represent hydrogen, C1-6 alkyl, hydroxyl, C1-6 alkoxy, etc., and R<;11>; and R<;12>; or R<;12>; and R<;13>; may bond together to form a 5- to 8-membered heterocycleÜ, etc.ü or a salt thereof.