An antitumor agent for treating a malignant tumor patient expressing EGFR having at least one mutation selected from the group consisting of G719X mutation of exon 18, E709X mutation of exon 18, and L861X mutation of exon 21, wherein X represents an arbitrary amino-acid residue, the antitumor agent comprising (S)-N-(4-amino-6-methyl-5-(quinolin-3-yl)-8,9-dihydropyrimido[5,4-b]indolizin-8-yl)acrylamide or a salt thereof.