A 68-year-old man was admitted to our hospital for investigation of dysuria and palpable lower abdominal mass. Computed tomography and magnetic resonance imaging demonstrated a huge bladder diverticulum tumor which infiltrated the mesentery and abdominal wall with multiple lymph node swelling in the pelvis. The tumor was partially resected by a transurethral approach and was histologically diagnosed as malignant neoplasm with marked necrosis. The patients underwent partial cystectomy with dissection of the mesentery and ileum. Pathological examination showed the tumor to be a leiomyosarcoma with mesentery infiltration. The patient had local recurrence with pulmonary metastasis two weeks after the operation and died 39 days after the operation. This is the third case of leiomyosarcoma of the diverticulum of urinary bladder reported in Japan.