Conventional gastrojejunostomy has been employed for unresectable advanced gastric cancer with pyloric stenosis; however, it is often not fully effective. We report a patient with unresectable gastric cancer who was effectively treated with an anticancer drug, S-1, after receiving an improved gastrojejunostomy. The patient was a 55-year-old woman who was referred to our hospital for epigastric pain. Upper gastrointestinal endoscopy showed a Borrmann III tumor in the antrum of the stomach, and gastric roentgenography showed pyloric stenosis. Preoperative findings were T3N2H0P0, stage III b. At operation, the tumor was found to have invaded the duodenum and the head of the pancreas, and disseminated nodules were found in the mesenterium of the small intestine, the left diaphragm, and the round ligament of the liver. A curative operation was impossible for the advanced gastric cancer. Therefore, an improved gastrojejunostomy was performed to allow oral intake. Oral intake started 7 days after the operation, and she left our hospital 20 days after the operation. She started treatment with 80 mg/day of S-1, given orally, for 28 days, followed by 14 days' rest, as 1 course. During 16 courses of the treatment, she maintained a performance status of 0 to 1 and maintained quality of life. However, she died because of pelvic dissemination and genital bleeding (caused by tumor invasion into the uterus) 2 years and 4 months after the surgery. This case suggested that the improved gastrojejunostomy was a useful method for treating unresectable gastric cancer, allowing the possibility of oral intake, and the use of S-1.