We report a case in which mucinous adenocarcinoma arose in a duplication cyst at the distal ileum with in-traperitoneal seeding. A thirty-three-year-old male patient presented with abdominal distension. Ultrasonography, CT and MR imaging revealed a dumbbell-shaped cystic mass adherent to the small intestine. The wall of the mass was thickened in two areas and contained inhomogeneous materials. A large amount of ascites with irregular masses along the greater omentum were seen present. Surgery revealed a duplication cyst adherent to the ileum. Pathologic examination proved that the thickened portions of the wall of the mass were mucinous adenocarcinoma, and that the nodules on the greater omentum were metastatic adenocarcinoma.