Pancreas transplantation has the highest surgical complication rate of all routinely performed organ transplantation procedures. The complications are not only caused by the pancreas itself but also occur due to issues with the transplant recipient. We report the case of a patient who experienced massive gastrointestinal bleeding after simultaneous pancreas-kidney transplantation (SPK), which was stopped successfully using somatostatin analog.The patient was a 45-year-old woman with diabetes mellitus type 1 who underwent SPK with enteric drainage. She had melena 5 days after SPK.At first, we suspected that the melena was caused by the transplanted duodenum because of rejection and ischemic changes. The patient experienced severe bleeding 9 days after SPK. We quickly performed open surgery and inserted an endoscope from the recipient's ileum to investigate the transplanted duodenum. However, no bleeding source was found, including in the transplanted duodenum and the recipient's ileum end. We determined that the bleeding source was the recipient's ascending colon. We attempted to perform endovascular treatment but could not detect the source of the bleeding; therefore, we used somatostatin analog to let the blood vessels shrink and reduce pancreatic output. Thereafter, the function of the transplanted pancreas and kidney gradually recovered, and the recipient was discharged 154 days after SPK.Gastrointestinal bleeding is a lethal complication and has several different causes, such as mucosal rejection, ischemic changes, and exocrine output of the pancreas graft. Somatostatin analog is one of the most acceptable treatments for patients who have gastrointestinal bleeding after SPK.