There are few reports of resected cases of second primary lung cancer in post-treatment survivors of small-cell lung cancer. Here, we report a surgical case of a 62-year-old female with second primary lung adenocarcinoma after chemoradiotherapy against small-cell lung cancer. She had been treated for small-cell lung cancer 2 years earlier, and achieved complete response after the treatment. A new nodular lesion was detected at a different segment in the right lower lobe. We performed a right lower lobectomy accompanied with systemic mediastinal nodal dissection. Histopathological findings revealed that the new nodular lesion was a second primary lung adenocarcinoma. No metastatic tumor was seen in the dissected lymph node; the initial tumor had disappeared completely. The postoperative course was uneventful, and she was discharged on day 10 after the operation. Ten months after the operation, she was free of recurrent tumor.