A 56-year-old woman was diagnosed with rectal cancer and liver metastases(Stage IV), and underwent low anterior resection and laparoscopic partial hepatectomy. The patient received adjuvant chemotherapy(mFOLFOX6 for 24 weeks), but developed multiple lung metastases 11 months later. Before undergoing a pulmonary resection, the patient presented with acute small bowel obstruction. Abdominal computed tomography showed small bowel stenosis due to a tumor, and we suspected peritoneal metastases from the rectal tumor. We performed partial resection of the small intestine, and histopathological examination revealed a primary small bowel tumor. The patient was discharged to her home without complications, and later underwent pulmonary resections for bilateral lung metastases. We usually suspect that small bowel obstruction is due to peritoneal metastases in patients with advanced colorectal tumors, but must consider the rare possibility of a separate primary small bowel tumor, especially in patients with a solitary lesion. We report a rare primary small bowel tumor after FOLFOX treatment in a patient with Stage IV rectal cancer.