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 1 -L 2 -L P -NH- (CH 2 ) n 1 -L a -(CH 2 )n 2 -C(=O)- wherein the anti-HER2 antibody is connected to the terminal L 1 , and the antitumor compound is connected to the carbonyl group of the -(CH 2 )n 2 -C(=O)-moiety with the nitrogen atom of the amino group at position 1 as the connecting position.
