A 62-year-old woman was referred for cough and lower abdominal pain. 18F-FDG PET/CT showed strong uptake not only in the left lung mass and hilar and mediastinal lymph nodes, but also a huge lower abdominal mass. All lesions were initially thought to be multiple metastases because bronchial biopsy of the lung mass showed poorly differentiated adenocarcinoma. However, the abdominal mass was found to be malignant peritoneal mesothelioma after surgical resection. It was difficult to diagnose this case correctly before resection because localized malignant peritoneal mesothelioma is rare.