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: -L1-L2-LP-NH—(CH2)n1-La-(CH2)n2-C(═O)— wherein the anti-HER2 antibody is connected to the terminal L1, and the antitumor compound is connected to the carbonyl group of the —(CH2)n2-C(═O)— moiety with the nitrogen atom of the amino group at position 1 as the connecting position.