A 68-year-old man with dyspnea and chest pain on left anterior chest during inspiration visited our hospital. An anterior mediastinal mass was detected on a chest computed tomography (CT) and we considered performing surgical treatment. Since the beta-human chorionic gonadotropin (beta-hCG) level was increased to 20.46 mIU/ml on the preoperative blood test, incisional biopsy was performed through a Chamberlain incision to rule out the mediastinal germ cell tumors. After diagnosing a benign mass on the postoperative pathological examination of the incisional biopsy specimen, total thymectomy that included the mass was performed via a full sternotomy. On the pathological examination after the second operation, the tumor was diagnosed as thymic adenocarcinoma producing beta-hCG, and the tumor had originated from the thymus.