A 61-year-old man consulted our hospital because of bloody sputum. Cells of Class V (adenocarcinoma) were found on sputum cytologic examination. Chest computed tomography (CT) showed reticular shadows but no obvious mass was detected. Positron emission tomography with 18-fluorodeoxyglucose (FDG-PET) revealed FDG uptake in both lower lung fields and more increased FDG uptake in a small area of the left lung field. Repeated chest CT, bronchial brushing, bronchial washing, and lung-imaging fluorescence endoscopy were performed, however, resulting in no detection of the primary site of lung cancer. Six months after initial consultation, chest CT showed an enlargement of the shadow in left S6 corresponding to the area of the more increased FDG uptake in PET. On the other hand, the shadow in right S10 did not change in size. Bronchial brushing of the left S6 was performed again, and class IV cells (adenocarcinoma) were found. After left lower lobectomy, diagnoses of well differentiated adenocarcinoma and usual interstitial pneumonia (UIP) were established histologically. There has been no report demonstrating the efficacy of FDG-PET for diagnosis of lung cancer with idiopathic pulmonary fibrosis (IPF).However, combination of lung cancer should be considered if PET shows spots of high FDG uptake in the lung fields of IPF.