The purpose of this study is to conduct a study on genetic pathology, obtaining of pure cancer tissues is mandatory. Although the endoscopy with biopsy has been a gold standard for diagnosing gastric cancer, percentage of cancer cells in biopsy samples is usually 30% or less. Recently, confocal laser endomicroscopy has been introduced for real-time histopathologic diagnosis in various cancers. The investigators aim to evaluate that biopsy using probe-based confocal laser endomicroscopy will increase the percentage of cancer cells in biopsy samples.

Inclusion Criteria:

1. Age, between 20 and 80
2. Initial-onset gastric cancer which would be treated with surgery

Exclusion Criteria:

1. Previous subtotal gastrectomy
2. Pregnancy or breast milk feeding
3. Active infection
4. Significant cardiopulmonary disease
5. Active hepatitis or severe hepatic dysfunction
6. Severe renal dysfunction
7. Severe bone marrow dysfunction
8. Severe neurologic or psychotic disorder