group r 2 represents an alkyl group an aryl group or a hydrogen atom r 3 represents a hydrogen atom or an alkyl group r 4 r 5 r 6 and r 7 each represents a hydrogen atom an alkyl group an aryl group a halogen atom or 21 nr 21 r 22 nso 2 r 22 ncor 22 and coor 21 and r 21 and r 22 each represents a hydrogen atom an alkyl group and an aryl group provided that the compound represented by formula does not have a carboxy group or a salt thereof as a substituent str3 wherein r 2 r 3 r 4 r 5 r 6 and r 7 each represents the same groups as those defined for r 2 r 3 r 4 r 5 r 6 and r 7 in formula x represents so 2 nh so 2 nhco conhco or so 2 nhso 2 r 8 represents an alkyl group an aryl group or a heterocyclic group r 9 represents a hydrogen atom an alkyl group an aryl group or 23 nr 23 r 24 ncor 24 or coor 23 and r 23 and r 24 each represents a hydrogen atom an alkyl group and an aryl group provided that the compound represented by formula does not have a.