An antitumor agent for treating a patient suffering from a malignant tumor expressing EGFR that has an exon 20 insertion mutation, said antitumor agent comprising a compound selected from the group consisting of compounds A to D described in the description or a salt thereof.本發明是一種用以治療惡性腫瘤患者之抗腫瘤劑,該惡性腫瘤患者表現出具有外顯子20插入突變之EGFR,該抗腫瘤劑含有選自於由說明書記載之化合物A~D所構成群組之化合物或其鹽。