A 70-year-old woman was admitted to our hospital with pleuritis and pericarditis. Cytological examination of pleural and pericardial effusion, and pleural biopsy specimens under thoracoscopy revealed no specific pathological findings. The pleural effusion was drained continuously; however, she died of circulatory insufficiency at day 45 from admission. At autopsy, a fragile hemorrhagic mass arising from the right auricle had invaded bilateral pleura and the pericardium directly without distant metastasis. Immunohistochemical staining showed that the tumor cells expressed endothelial markers such as CD31 and CD34 antibodies, and factor VIII-related protein. These findings supported the diagnosis of a poorly differentiated angiosarcoma.