A 49-year-old woman underwent hysterectomy and bilateral adnexectomy after the diagnosis of a right ovarian tumor with paraaortic and pelvic lymph node metastases. The pathological diagnosis was undifferentiated carcinoma of the ovary. After the operation, a bladder tumor was discovered during the evaluation for microscopic hematuria. The bladder tumor was pathologically diagnosed as transitional cell carcinoma, pT1b, G3. Although the pathological findings of the bladder cancer and ovarian cancer were very similar, we could diagnose primary bladder cancer with ovary and lymph node metastases according to the immunohistochemical staining pattern of cytokeratins 7 and 20. Herein, the clinical usefulness of immunohistochemical staining using cytokeratins for making a differential diagnosis of the origin of a tumor in the pelvic cavity is demonstrated.