To demonstrate non-inferiority of CJ-12420 to lansoprazole 30 mg capsule in terms of therapeutic efficacy, and to confirm safety of CJ-12420, after once daily oral administration of CJ-12420 50 mg, 100 mg or Lansoprazole 30 mg capsule in gastric ulcer patients.

This is a double blind, randomized, placebo-controlled, phase 3 study. Subjects will be randomly assigned to one of the three treatment groups (CJ-12420 50 mg, 100 mg or Lansoprazole 30 mg).

Inclusion Criteria:

1. Diagnosis of 1 or more active gastric ulcers (≥3 mm ~ ≤30 mm using open biopsy forceps) according to the Sakita-Miwa classification (A1 or A2 stage) from upper GI endoscopy at the same institution within 14 days prior to initiation of the investigational product administration.

Exclusion Criteria:

1. Finding of gastrointestinal bleeding, esophageal stricture, ulcer stenosis, pyloric stenosis, esophageal gastric varices, Barrett's esophageal of >3 cm (long segment Barrett esophagus, LSBE), duodenal ulcer, intractable ulcer, digestive ulcer perforation or malignancy on upper GI endoscopy.
2. Ulcer caused by an endoscopic surgery (e.g., ulcer after EMR/ESD)
3. Requirement of persistent daily use of drugs that may cause an ulcer such as nonsteroidal anti-inflammatory drugs(NSAIDs) or aspirin during the course of the study
4. Scheduled surgery requiring hospitalization or requirement of surgical treatment during study participation
5. Subjects who participated in the other clinical trial within 4 weeks prior to randomization