Primary malignant tumors originating in the pulmonary artery are extremely rare. Recently, We experienced a case of primary leiomyosarcoma occurred on the main pulmonary artery and extended into the Rt and Lt pulmonary artery. The patient was 44 year-old woman and the chief complaint was severe exertional dyspnea. Emergency pulmonary arteriotomy to relieve the pulmonary artery obstruction was performed on cardiopulmonary bypass. The tumor was 6 x6 x4 cm in size and infiltrated into the main pulomary artery. The tumor in the pulmonary artery was removed without any difficulties. The patient was recovered without any specific problems. The tumor was confirmed to be leiomyosamoma histopathologically.