The purpose of this study is to determine the efficacy of TAK-438, once daily (QD), compared to lansoprazole in patients with duodenal ulcer.

Inclusion Criteria:

1. Participants must have endoscopically confirmed duodenal ulcers (mucosal defect with white coating) .At least one ulcer with white coating of 5 mm or larger in size should be observed at baseline (Visit 1).
2. Outpatient (including short inpatient for examination and others)

Exclusion Criteria:

1. Participants with a gastric ulcer which is suspected to be malignant on endoscopy at baseline (Visit 1)
2. Participants with an Acute Duodenal Mucosal Lesion (ADML) on endoscopy at baseline (Visit 1)
3. Participants with a linear ulcer (including scarring) on endoscopy at baseline (Visit 1)
4. Participants with a postoperative ulcer (e.g., Ulcer after EMR/ESD) on endoscopy at baseline (Visit 1)
5. Participants with a gastric ulcer on endoscopy at baseline (Visit 1)
6. Participants with an ulcer for which medical treatment is not indicated (e.g.,perforation, pyloric stenosis, duodenal stenosis, large hemorrhage)
7. Participants who have received endoscopic hemostasis for gastric ulcer within 30 days prior to baseline (Visit 1)
8. Participants with a previous or current history of Zollinger-Ellison syndrome, or other gastric acid hypersecretion disorders
9. Participants who have received or who are scheduled to undergo surgery which affects gastric acid secretion (e.g., resection of upper gastrointestinal tract, vagotomy)