A 72-year-old man visited our outpatient clinic because of a duodenal subepithelial tumor (DSET) seen on upper gastrointestinal endoscopy during a medical checkup at another hospital. The DSET was an incidental finding during an upper GI endoscopy procedure performed on the patient 6 years ago, and since then, the patient has been regularly followed-up at local clinics. He had a history of abdominal surgery that included a cholecystectomy performed 30 years ago because of abdominal trauma. There were no special findings on physical examination, and the patient had no upper abdominal symptoms. The results of his laboratory test, which included tests for tumor markers, were normal. On endoscopy, a round, elevated mass, approximately 2 cm in size, was found in the duodenal bulb. On comparing the current size of the lesion on endoscopy 6 years ago, it was suspected that the lesion had increased in size. We performed EUS and contrast-enhanced computed tomography (CT) to identify the tumor. On EUS, an 18 mm hypoechoic mass was found in the muscularis propria layer of the duodenal wall. On CT, a 1.4 cm mass was observed near the duodenal wall and the cystic duct stump, and a round cyst was seen along the side. It was difficult to determine if the lesion originated from the duodenal wall or from the cystic duct. Although the patient did not have any symptoms, such as pain, jaundice, or weight loss, and the results of laboratory tests, which included tests for tumor markers, were normal, we decided to surgically resect the tumor because serial follow-up endoscopy showed that the tumor had increased in size.