Concurrent chemoradiotherapy (CRT) is the standard of care for locally advanced non–small cell lung cancer (LA-NSCLC). However, this intensive therapy often causes severe esophagitis, which could deteriorate a patient's quality of life (QOL), leading to poor treatment compliance. Sodium alginate, approved in Japan for gastritis, is sufficiently highly viscous to remain in the esophageal mucosa, providing a protective effect in the esophagus. To investigate whether this compound has a preventive effect against severe esophagitis in patients receiving concurrent CRT, we plan a 3-arm randomized trial of sodium alginate with 2 different schedules versus water. The primary endpoint is set as the proportion of patients with grade ≥ 3 esophagitis using the Common Terminology Criteria for Adverse Events, version 4.0. With stratification by institute, performance status, and percentage of the esophageal volume receiving >35 Gy, the patients will be randomly assigned to 1 of the following groups: sodium alginate initiated concomitantly with CRT (group A), sodium alginate initiated soon after the development of extremely mild esophagitis during CRT (group B), or water administered throughout CRT (group C). Assuming that the proportion of grade ≥ 3 esophagitis would be 8% in groups A and B and 27% in group C, the required sample size would be 200 patients, with 70% power and 5% α. The secondary endpoints include QOL, the frequency of additional prescriptions of analgesics, treatment response, and survival. The results of the present study will clarify whether sodium alginate can prevent esophagitis in patients with LA-NSCLC undergoing CRT.