A 77-year-old man was admitted to our hospital complaining of gross hematuria. Cystoscopy showed an approximately 4-cm non-papillary tumor in and out of the diverticulum of the left posterior wall. Total cystectomy was performed. Histopathological diagnosis was pleomorphic leiomyosarcoma. According to TNM classification of bladder cancer, the stage of this tumor was pT3bpN0M0. The patient had local recurrence two months after the operation, and died a month later. This is the second case of leiomyosarcoma of the diverticulum of urinary bladder reported in Japan.