Abstract A 54-year-old man presented at the Mibayashi clinic with bowel discomfort. Ultrasonography showed a left renal mass and the patient was referred to Noto General Hospital for urological evaluation. Results of the physical examination were unremarkable, but computed tomography and magnetic resonance imaging demonstrated a large tumor in the middle of the left kidney. The patient underwent left nephrectomy. On hemisection of the kidney, a firm tumor, measuring 8 × 7.5 cm, was seen occupying the renal pelvis. The histological diagnosis was leiomyosarcoma arising from the right renal pelvis. No treatment was provided after surgery and no recurrence was observed 6 months postoperatively.