Provided is a therapeutic agent for treating breast cancer that has developed resistance to administration of CDK4/6 inhibitors and estrogen antagonists, the agent containing 5-((2-(4-(1-(2-hydroxyethyl)piperidin-4-yl)benzamide)pyridin-4-yl)oxy)-6-(2-methoxyethox y)-N-methyl-1H-indole-1-carboxamide or a pharmacologically acceptable salt thereof.