A process for preparing a compound represented by the formula (I): comprising reacting a compound represented by the formula (II) or salt thereof: with a compound represented by the formula (III): in the presence of a condensation reagent, wherein R 1 represents 1) optionally substituted azetidin-1-yl, 2) optionally substituted pyrrolidin-1-yl, 3) optionally substituted piperidin-1-yl, etc.； R 2 , R 3 , R 4 and R 5 may be the same or different and each represents hydrogen or fluorine； and R 6 represents hydrogen or fluorine.
