An anti-tumor agent for treating patients with malignant tumors that express EGFR having at least one mutation selected from the group consisting of exon 18 G719X mutations, exon 18 E709X mutations, and exon 21 L861X mutations, the anti-tumor agent containing (S)-N-(4-amino-6-methyl-5-(quinolin-3-yl)-8,9-dihydropyrimido[5,4-b]indolizin-8-yl)acrylamide or a salt thereof. [X represents a discretionary amino acid residue.]
