Immuno-checkpoint inhibitors (ICI) have become an effective treatment option for non-small-cell lung cancer patients. However, ICI therapy was reported to be less effective in patients with epidermal growth factor receptor (EGFR) mutations than in those with wild-type EGFR. We report here that an non-small-cell lung cancer patient with the EGFR mutant T790M showed a programmed cell death ligand 1 (PD-L1) expression level that increased from <25% to >90% after eighth-line osimertinib therapy. He was treated with pembrolizumab as a ninth-line treatment, and attained stable disease. After the pembrolizumab therapy, he was treated with gemcitabine, which produced a good response despite being the 10th-line treatment. We should consider administering ICI and chemotherapy even to EGFR mutant patients after failure of EGFR tyrosine kinase inhibitor, especially in cases with high PD-LI expression.