l group, or R12 and R13 are optionally bonded to form a ring); and [0018] R6 and R7 [0019] are the same or different and each is a hydrogen atom, an optionally substituted hydrocarbon group, or R6 and R5 are optionally bonded to form a ring, [0020] or a salt thereof (hereinafter sometimes to be referred to as compound (II)); [0021] (2) the inhibitor of the above-mentioned (1), which contains E-4-[