(EN) x1d2d4d6dA process for preparing a compound represented by formula (Y1) or (Y2) [wherein R is an optionally substituted carbocyclyl lower alkyl group or the like] using a novel process for preparing a pyridone derivative represented by formula (X4) [wherein R is hydrogen halogen or the like; R is hydrogen a lower alkyl group or the like said lower alkyl group being optionally substituted with one or more selected from substituent group E; R is a lower alkyl group or the like said lower alkyl group being optionally substituted with one or more selected from substituent group E; and R is a lower alkyl group or the like said lower alkyl group being optionally substituted with one or more selected from substituent group E].