A 76-year-old man taking anti-tuberculous drugs for active pulmonary tuberculosis was admitted to our hospital requiring further examination of a pulmonary mass in the left upper lobe. He had had right lower lobectomy and partial middle lobectomy for non-small-cell lung cancer two years before admission. Retrospectively, the left upper lobe mass had been gradually growing for about 2 months. We performed a transbronchial lung biopsy of the pulmonary mass, and diagnosed small-cell lung cancer. This report describes a rare case of small-cell lung cancer appearing during treatment for active pulmonary tuberculosis after non-small-cell lung cancer surgery.