Neuron protective agents on the basis of the inhibition of neuronotoxicity of kainic acid and compounds useful as such agents. Pyridothiazine derivatives represented by general formula (I) or pharmaceutically acceptable salts thereof and neuronotoxicity inhibitors containing as the active ingredient these compounds or pharmaceutically acceptable salts thereof, wherein A represents a pyridine ring; formula (II) represents formula (III) or formula (IV); and R 1 , R 2 , R 3 , R 4 and R 5 are the same or different and each represents hydrogen, optionally substituted lower alkyl, cycloalkyl, alkenyl, aryl, carboxy or lower alkoxycarbonyl or does not exist, or R 2 and R 3 together form a nitrogenous heterocycle optionally having a nitrogen atom as another heteroatom, being fused with a benzene ring, or being substituted by lower alkyl.