A patient with primary cardiac lymphoma, which is very rare, generally is regarded to have a poor prognosis. We herein report a patient with cardiac lymphoma who was treated successfully by systemic chemotherapy and radiotherapy using a pacemaker to control the complete atrioventricular (A-V) block. A 70-year-old man had a syncope caused by a complete A-V block. An echocardiogram, a computed tomographic scan, and magnetic resonance imaging of his chest showed a cardiac tumor. At this time, a biopsy of the cardiac tumor disclosed malignant lymphoma (diffuse large cell type, B cell type). The patient was thus treated with systemic chemotherapy and radiotherapy and, as a result, achieved a complete remission with a disappearance of the A-V block. Recently, several successful outcomes involving primary cardiac lymphoma have been reported because of the progress in diagnostic techniques including echocardiography, computed tomographic scanning, and magnetic resonance imaging, as well as improvement in the therapy of malignant lymphoma. Our clinical experience indicated that an early and accurate diagnosis combined with the appropriate therapy can thus help in obtaining a long survival in patients with primary cardiac lymphoma.