A case of superficially infiltrating cardiac angiosarcoma damaging the right coronary artery and forming a pseudoaneurysm was studied. The patient was a 25-year-old male who had undergone cardiac tamponade twice in the past six months. Since heart murmur was audible in auscultation, thorough examination was performed. Cardioechography and cardiac angiography revealed a pseudoaneurysm of hen egg size involving the proximal right coronary artery. Although the pseudoaneurysm and right atrium ruptured during operation, a coronary bypass was placed and the right atrium was repaired with an equine pericardium. Angiosarcoma was identified by pathological examination of the excised specimen. The patient died two months after operation due to lung metastasis.