The patient was a 67-year-old man who had begun experiencing abdominal distention in August 2012. When concomitant dorsal pain developed, he visited a physician. A large volume of ascites was visible during an abdominal ultrasonography, and the patient was referred to the Department of Gastrointestinal Medicine of our hospital. CT showed a 6-cm ischemic mass accompanied by calcification directly below the lower median abdominal wall, and bladder infiltration was suspected. Moreover, a large volume of ascites and nodes in the greater omentum was noted. FDG-PET showed accumulation in the greater omentum, but accumulation in the mass was mild. Cystoscopy showed only exclusion by the tumor. On the basis of these findings, the physicians suspected urachal cancer accompanied by peritoneal dissemination, and the patient was referred to our department for excisional biopsy. The abdominal cavity contained 3,500 ml of yellow ascites, and many peritoneally disseminated nodes were present in the greater omentum and mesentery. The tumor was continuous with the bladder through infiltration. The greater omentum and a part of the bladder were removed by combined resection. The postoperative recovery was smooth, and the patient was discharged on day 8. Histopathological examination showed that the tumor was mucinous adenocarcinoma and was continuous with the residual urachus below the bladder mucosa. Thus, the tumor was diagnosed as urachal cancer. The patient is now under systemic chemotherapy treatment with mFOLFOX6+bevacizmab.