There are several causes of hypotension during anesthesia. We report a case of severe hypotension caused by external cardiac compression. A 72-year-old man was scheduled for resection of mediastinal tumor under general anesthesia. He had undergone mediastinal tumor resection four times uneventfully. Anesthesia was induced and maintained with target controlled infusion of propofol and continuous infusion of remifentanil. Tracheal intubation was facilitated with rocuronium. Massive bleeding and severe hypotension developed during the operation. Blood transfusion, cryoprecipitate, fresh frozen plasma, and percutaneous cardiopulmonary support were commenced. However, hemorrhage was not the only cause of hypotension. The transesophageal echocardiography revealed external cardiac compression by tumor and doctor's hand. Transesophageal echocardiography was useful for verifying the causes of hypotension. It is necessary to evaluate the causes of hypotension during the operation, because a certain number of problems may exist.