Mesothelioma is a rare aggressive tumor arising from the mesothelial cell and regarded as universally fatal disease with average survival around 1 year. The incidence rate is varied from one to forty per million in different countries and increasing by the year. The most common site of tumor origin is the pleura and only 20% to 33% of mesothelioma arise from the peritoneum. There are increasing reports of malignant mesothelioma with forty to fifty fatal cases per year in Korea. Histological studies with immunohistochemical stain is helpful for the diagnosis of peritoneal mesothelioma and imaging modality alone is not sufficient for diagnosis, so it is difficult to confirm diagnosis. A 64-year-old male patient was admitted to the hospital with a palpable mass on abdomen. The 6x6 cm sized huge mass was seen on the body of stomach adjacent to the peritoneum. We report a case of malignant peritoneal mesothelioma without evident exposure to asbestos, of which direct invasion to the gastric mucosa was confirmed by endoscopic biopsy and immunohistochemical stain.