As a method for producing an antitumor drug which is excellent in terms of antitumor effect and safety and has an excellent therapeutic effect, there is provided a method for producing an antibody-drug conjugate comprising reacting a compound represented by the following formula: €ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ€ƒ(maleimid-N-yl)-(CH 2 )n 3 -C(=O)-L 2 -L P -NH-(CH 2 ) n 1 -L a -(CH 2 ) n 2 -C(=O)-(NH-DX) with an anti-HER2 antibody or a reactive derivative thereof, by forming a thioether bond at a disulfide bond site present in the hinge part of the antibody, in which -(NH-DX) is a group comprising an antitumor compound represented by the following formula:
