A 70-year-old woman, who had undergone a right radical mastectomy for breast cancer 27 years previously, was found to have a tumor measuring 15 mm in diameter between the S5 area and the S8 area of the left lung. We suspected it to be either metastatic or primary lung cancer based on preoperatively any conclusive diagnosis. We thus performed a thoracoscopic partial lung resection and pericardial resection. The pathological diagnosis of the resected specimen was pulmonary metastasis from the previous breast cancer, since the pathological findings of the lung lesion were closely similar to those of the previous breast lesion. Using immunohistochemical methods, the tumor cells show positive staining for anti-estrogen receptor antibody. In lung tumor cases in which the patient has undergone a breast cancer resection, even more than 20 years previously, surgeons must not rule out the possibility of recurrence. Thoracoscopic surgery is considered to be most effective method for both making a definitive diagnosis and for performing curative treatment.