We report a case of a 73-year-old man diagnosed with pulmonary pleomorphic carcinoma who showed profound durable response after a single treatment with pembrolizumab. The patient underwent a diagnostic workup in our hospital due to a hoarseness of voice. Chest computed tomography revealed a massive pulmonary tumor in the left upper lobe and multiple nodules in the both lung fields. Histological examination of a transbronchial lung biopsy specimen revealed pulmonary pleomorphic carcinoma. First-line treatment with pembrolizumab was discontinued after a single administration due to treatment-related pneumonitis. However, durable response has been observed over 17 months to date.