A 51-year-old man was admitted with chief complaints of upper abdominal pain and vomiting. He was diagnosed with ileus caused by a small intestinal tumor on abdominal CT. Primary small intestinal cancer or gastrointestinal stromal tumor (GIST) was suspected, and laparoscopically assisted surgery was performed. Intraoperative findings revealed a 5-cm diameter tumor in the ileum, about 50 cm on the oral side of Bauhin's valve. Tumor excision through a small incision was considered possible, so we performed partial excision of the small intestine. An annular stricture-type tumor was observed in the resected specimen. Histopathological examination diagnosed small intestine mucinous adenocarcinoma, and metastasis to the lymph nodes near the tumor was observed. The tumor was defined as pT3 (SS), ly(+), N1 (1/4), stage IIIA (UICC, seventh edition). We encountered a rare case of primary small intestine mucinous adenocarcinoma that was discovered because of ileus symptoms and was excised under laparoscopic assistance, and report it here with some discussion of the literature.