Dermatomyositis is an uncommon in flammatory myopathy with characteristic cutaneous manifestations which is frequently linked to several cancers. A 42-year-old man presented with left flank pain with typical symptoms of dermatomyositis. Computed tomography showed a solid mass in the pelvis of the left kidney and lymphadenopathy in the retroperitoneum. Since the general condition of the patient rapidly deteriorated because of marked muscle weakness of the respiratory muscles, the patient initially underwent medical treatment with prednisolone. After the improvement of respiratory function, the patient underwent radical nephroureterectomy and retroperitoneal lymphadenectomy. The pathological specimen showed moderately differentiated adenocarcinoma, stage pT3N2. After surgery, the patient showed a marked improvement of clinical symptoms related to dermatomyositis. Twenty-two months later, multiple organ metastases occurred and the patient died of cancer. We should point out the significance of surgical and medical treatment for these patients.