Inflammatory myofibroblastic tumor (IMT), which expresses cyclooxygenase-2 (COX-2), can be effectively treated with COX-2 inhibitor. Here, we report a case of urinary bladder IMT in a 13-year-old boy. Although total cystectomy was initially planned for complete resection of the tumor, neoadjuvant treatment with COX-2 inhibitor and prednisolone reduced the size of the tumor and enabled complete resection of the tumor by partial cystectomy. Neoadjuvant treatment with COX-2 inhibitor and prednisolone for IMT of the bladder allowed a more conservative surgical procedure that preserved bladder function.