A dibenzothiazepine compound is suitably prepared by subjecting a 2-amino-2′-carboxy-diphenylsulfide compound to dehydration-condensation reaction in the presence of an acidic catalyst; the 2-amino-2′-carboxy-diphenylsulfide compound is suitably prepared by reducing a 2-nitro-2′-carboxy-diphenylsulfide compound in a lower aliphatic ester solvent; and the 2-nitro-2′-carboxy-diphenylsulfide compound is suitably prepared by reacting a nitrobenzene compound with a thiosalicylic acid compound in a mixture of a lower aliphatic alcohol and water.