As treatment effective for HER2-expressing cancer having resistance or refractoriness to an existing anti-HER2 drug, there are provided a therapeutic agent and a treatment method using an antibody-drug conjugate in which a linker and a drug represented by the formula: - (Succinimid-3-yl-N)-CHCHCHCHCH-C(=O)-GGFG-NH-CH-O-CH-C(=O)-(NH-DX) are conjugated to an anti-HER2 antibody.