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