A 45-year-old woman was admitted to our hospital because of lower abdominal pain and anorexia. A barium gastrography and gastroscopy showed a type 4 gastric cancer in the upper gastric body. Histologic study on biopsy specimens from the tumor revealed poorly differentiated adenocarcinoma. Computed tomography revealed bilateral hydronephrosis, and barium enema showed diffuse stenosis of the sigmoid colon because of peritoneal dissemination. This patient was treated by intra-aortic infusion therapy with sequential MTX and 5-FU. After five courses of the administration, barium enema revealed reexpansion of the lumen of sigmoid colon with normalization of the tumor markers. The patient was discharged without symptoms. Intra-aortic infusion therapy with sequential MTX and 5-FU was considered an effective treatment for unresectable gastric cancer.