We report a case of a 64-year-old woman with an anterior mediastinal tumor on a chest computed tomography (CT) before operation of uterine cancer. After the radical surgery and a chemotherapy for uterine cancer, surgical resection of mediastinal tumor was performed in July 2005 because of gradually progression in tumor size. Under the median sternotomy, the tumor revealed intensive adhesion to the right lung and the left brachiocephalic vein. We performed a complete resection of the tumor with combined resection of adherent parts of them. The tumor was 9 x 8 cm in size, containing with yellow cream-like fluid and hair. Histologically, the tumor was diagnosed as a mature teratoma with the tissue of bone, digestive tract epithelium, bronchial epithelium and so on. In benign teratoma, it is not rare to perforate to the adjacent structures. So, we concluded if the mediastinal teratoma was suspected, long-term observation had the risk for adhesion and perforation to adjacent tissue, and in case of operation, we should keep in mind the possibility of combined resection of involved organs.