A 66-year-old Japanese man with recurrent adenocarcinoma of the lung p-stage IIIA (pT2bN2M0; version 8) on pembrolizumab was present with gradually worsening dyspnea. Although history and physical examination were unremarkable, high-resolution CT showed the perilymphatic distribution of the pembrolizumab-induced pneumonitis. Consistent with the CT result, biopsy revealed the aggregation of the cytotoxic (CD8+) T-lymphocytes around the lymph tracts. Given the clinical, radiological and pathological findings, pembrolizumab-induced pneumonitis was confirmed. The patient was discharged after terminating the pembrolizumab with ameliorated symptoms. This report, in conjunction with existing literature, illustrates the wide variety of the pembrolizumab-induced pneumonitis and bolsters the current understanding of its pathophysiology.