Inclusion Criteria:

• Pathologically proven unresectable adenocarcinoma of stomach
• With uni-dimensionally measurable disease (at least longest diameter 2 cm on conventional CT scan, x-ray or physical examination, or 1cm on spiral CT scan)
• Age 18 to 70 years old
• Estimated life expectancy of more than 3 months
• ECOG performance status of 2 or lower
• Adequate bone marrow function(absolute neutrophil count [ANC] ≥1,500/µL, hemoglobin ≥9.0 g/dL,and platelets ≥100,000/µL)
• Adequate kidney function (serum creatinine < 1.5 mg/dL)
• Adequate liver function (serum total bilirubin < 2 times the upper normal limit (UNL); serum transaminases levels <3 times [<5 times for patients with liver metastasis] UNL)
• No prior chemotherapy but prior adjuvant chemotherapy finished at least 6 months before enrollment was allowed. (but, prior adjuvant chemotherapy with capecitabine or S-1 or camptothecin analogues was excluded)
• No prior radiation therapy for at least 4 weeks before enrollment in the study

Exclusion Criteria:

• Other tumor type than adenocarcinoma
• Central nervous system (CNS) metastases or prior radiation for CNS metastases
• Gastric outlet obstruction or intestinal obstruction
• Evidence of gastrointestinal bleeding
• The patient has bony lesions as the sole evaluable disease.
• Past or concurrent history of neoplasm other than stomach cancer, except for curatively treated non-melanoma skin cancer or in situ carcinoma of the cervix uteri
• Pregnant or lactating women, women of childbearing potential not employing adequate contraception

• Other serious illness or medical conditions

  • Unstable cardiac disease despite treatment, myocardial infarction within 6 months prior to study entry
  • History of significant neurologic or psychiatric disorders including dementia or seizures
  • Active uncontrolled infection
  • Other serious underlying medical conditions which could impair the ability of the patient to participate in the study
• Concomitant administration of any other experimental drug under investigation, or concomitant chemotherapy, hormonal therapy, or immunotherapy

• concomitant drug medication; The following drugs cause drug interaction with S-1.

  i. Warfarin, phenprocoumon: increase bleeding tendency ii. Increase blood concentration of phenytoin iii. sorivudine: inhibit DPD -> increase toxicity according to fluoropyrimidine iv. allopurinol : decrease activity of S-1