A 21-year-old man was referred to our hospital because of an abnormal shadow on a routine chest radiogram. Enhanced computed tomography showed an 83×74 mm mass in the anterior mediastinum, with invasion of the superior vena cava (SVC). Surgical resection with sternotomy was performed. Intraoperative temporary bypass grafting with a 5-Fr catheter was performed between the right brachiocephalic vein and right atrium. The mediastinal tumor was resected with the SVC, and SVC reconstruction with a 16 mm expanded polytetrafluoroethylene graft was performed. The bypass stabilized intraoperative vital signs and enabled safe completion of the operation. The pathological diagnosis was seminoma. SVC replacement combined with temporary bypass using a small diameter catheter is technically feasible and safe.