A 2-azidomethyl-5-phenyl-1H-2,3-dihydro-1,4-benzodiazepine derivative of Formula X ##STR14## wherein R.sub.1 represents hydrogen or lower alkyl, and the phenylene group A and the phenyl group B independently from each other each may be unsubstituted or be substituted by 1 to 3 substituents selected from the group consisting of halogen, lower alkylthio, lower alkoxy, lower alkyl, hydroxy, nitro and trifluoromethyl, or be substituted at two adjacent carbon atoms by methylenedioxy or ethylenedioxy, and optical isomers and pharmaceutically-acceptable acid addition salts thereof.