A 57-year-old female was diagnosed as having advanced lung adenocarcinoma. Combination chemotherapy using cisplatin plus pemetrexed was introduced. However, her disease progressed during the maintenance therapy with pemetrexed. Subsequent therapies could not control the disease. Later, it was found that her lung cancer harbored an epidermal growth factor receptor(EGFR)exon 20 insertion, D770_N771insSVD. Afatinib, a second-generation EGFR-tyrosine kinase inhibitor (TKI), was administered as the fourth-line treatment and reduced the size in several lesions. Although EGFR-TKIs are generally not recommended as the first-line treatment for lung cancers with EGFR exon 20 insertions, afatinib could be a useful therapeutic option in the second-line or later therapy for this type of exon 20 insertion, D770_N771insSVD.