Inclusion Criteria:

1. Histological diagnosis of pancreatic adenocarcinoma.

2. Measurable or assessable lesions(Except for Phase I).

3. Age: 20 ~ 75 years.

4. Kar-nofsky Performance Status (KPS) > 70.

5. No prior chemotherapy

6. No history of treatment with gemcitabine or S-1.

7. No history of radiotherapy to the abdomen.

8. Oral intake of S-1 is possible.

9. Adequate function of major organs (bone marrow, heart, lungs, liver etc.). WBC
3,500/mm3 and Neutrophil 2,000/mm3. Hb 9.0 g/dl. Platelet count 100,000/mm3. GOT
and GPT 2.5 times the upper limit of normal (excluding liver metastasis). T-Bil
2.0 mg/dl. Creatinine within the upper limit of normal). Normal ECG (not
considering clinically unimportant arrhythmias and ischemic changes).

10. Predicted survival for >3 months.

11. Able to give written informed consent.

Exclusion Criteria:

1. Severe pleural effusion or ascites.

2. Metastasis to the central nervous system (CNS).

3. Active gastrointestinal bleeding.

4. Active infection.

5. Liver cirrhosis

6. Uncontrolled ischemic heart disease.

7. Serious complications (such as intestinal paralysis, intestinal obstruction,
interstitial pneumonia or pulmonary fibrosis, uncontrolled diabetes mellitus, heart
failure, renal failure, or hepatic failure).

8. Active multiple cancer.

9. Severe mental disorder.

10. Pregnancy, possible pregnancy, or breast-feeding.

11. Flucytosine treatment

12. Judged to be ineligible for this protocol by the attending physician.