A 72-year-old woman with a history of surgery for left breast cancer was found to have sigmoid colon cancer and solitary pulmonary tumor of left upper lobe. We diagnosed adenocarcinoma of the unknown origin by a transbronchial biopsy. We performed left upper segmentectomy and sigmoidectomy. Left pulmonary tumor was diagnosed metastatic lung tumor from breast cancer. A right pulmonary tumor was confirmed by chest computed tomography(CT) after sigmoidectomy. It was also considered to be metastasis from breast cancer and treated with vinorelbine ditartrate. Since no effect was observed by chemotherapy, tumor was surgically removed by wedge resection. Right pulmonary tumor was pathologically diagnosed as metastasis from sigmoid colon cancer. In suspicious case of pulmonary metastases from double cancer, the possibility of different lesions from different primary site should be kept in mind.