The purpose of this study is to evaluate the efficacy of TAK-438, once daily (QD) compared to AG-1749 (Lansoprazole) in patients with a history of gastric or duodenal ulcer who require long-term therapy of low-dose aspirin.

Inclusion Criteria:

1. Participants who require continuous low-dose aspirin therapy during the treatment period with the study drug
2. Participants who have a history of ulcer in stomach or duodenum, endoscopically confirmed
3. Outpatient (including inpatient for examinations)

Exclusion Criteria:

1. Participants scheduled to change the type and dosage regimen of low-dose aspirin
2. Participants with ulcers or bleeding in stomach or duodenum, endoscopically confirmed
3. Participants with small intestine bleeding, large intestine bleeding, or gastrointestinal bleeding of unknown etiology
4. Participants who have a history of surgery which affects gastric acid secretion (e.g., resection of upper gastrointestinal tract, vagotomy) or who are scheduled to undergo such surgery
5. Participants with a previous or current history of Zollinger-Ellison syndrome, or other gastric acid hypersecretion disorders
6. Participants with a previous or current history of aspirin-induced asthma