The patient was 48-year-old woman. The patient was referred to our hospital due to detection of an abnormal shadow on a chest X-ray image during a health checkup. The results of a CT scan morphologically suggested a benign tumor, but a diagnosis could not be made even after bronchoscopic cytology. FDG-PET revealed accumulation at the tumor site suggestive of malignancy, and an operation was performed. Prompt pathological examination showed that the tumor was adenocarcinoma, and right lower lobectomy and lymphnode dissection were performed. FDG-PET is thought to be useful for diagnosis in cases of lung tumor in which a preoperative diagnosis can not be established.