Clinically highly useful drugs having an antagonism to serotonin, in particular, central muscle relaxing drugs for treating, ameliorating or preventing spastic paralysis or ameliorating myotonia and comprising fused pyridine derivatives represented by general formula (I), pharmacologically acceptable salts thereof or hydrates of either (I) or (1), wherein the ring A represents a benzene, pyridine, thiophene or furan ring; and B represents (2).