Method of treating cancer comprising administration of anti-HER2 antibody-drug conjugate

12P21a22122 As an antitumor drug which is excellent in terms of antitumor effect and safety and has an excellent therapeutic effect, there is provided an antibody-drug conjugate in which an antitumor compound represented by the following formula is conjugated to an anti-HER2 antibody via a linker having a structure represented by the following formula: -L-L-L-NH-(CH)n-L-(CH)n-C(═O)- wherein the anti-HER2 antibody is connected to the terminal L, and the antitumor compound is connected to the carbonyl group of the -(CH)n-C(═O)- moiety with the nitrogen atom of the amino group at position 1 as the connecting position.