To provide an antitumor drug having excellent therapeutic effect, which is excellent in terms of antitumor effect and safety. Provided is an antibody-drug conjugate in which an antitumor compound represented by the following formula is conjugated to an anti-HER3 antibody via a linker having a structure represented by the formula : -L1-L2-LP-NH-(CH2)n1-La-(CH2)n2-C(=O)- or -L1-L2-LP- (the anti-HER3 antibody is connected to the terminal of L1, the antitumor compound is connected to the carbonyl group of -(CH2)n2-C(=O)- moiety or the C terminal of LP, with the nitrogen atom of the amino group at position 1 as a connecting position).