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)—;CH2CH2CH2CH2CH2—;C(═;O)-GGFG-NH—;CH2—;O—;CH2—;C(═;O)—;(NH-DX) are conjugated to an anti-HER2 antibody.