Diaphragmatic tumors, whether benign or malignant, may not generally reveal any symptoms in the early phase and may be found accidentally. During a pre-employment physical examination, a 20-year-old woman was found to have an abnormal shadow on the left diaphragm. An X-ray film, computed tomography and ultrasonography showed a giant mass on the left side, to the rear of the heart. She underwent surgery via a left thoraco-abdominal approach. The lesion was found to arise from the left diaphragm, and multiple disseminated lesions were scattered in the left thoracic cavity. Histological examination showed many large, oxyphilic rhabdoid cells between diffusely proliferating, spindle-shaped cells, and the tumor was subsequently diagnosed as a pleomorphic rhabdomyosarcoma of the diaphragm, of which the location and histological type were very rare. Despite adjuvant therapy, chest X-ray and CT revealed increasing tumor growth in the left cavity and she died one year after surgery.