Antibody-drug conjugate having improved stability and use thereof

An antibody-drug conjugate comprising a cytotoxic drug conjugated to the N-terminal amino acid residue of the heavy chain or light chain of an antibody,

• wherein the cytotoxic drug is site-specifically conjugated to the N-terminal α-amine group of native first amino acid of the heavy chain or light chain of the antibody, and
• the cytotoxic drug is conjugated to the antibody by a linker having a reactive aldehyde group, and the conjugate is produced through reductive alkylation by the aldehyde group.