To investigate the effect of irsogladine on radiotherapy-induced oral mucositis, a prospective randomized study of 43 patients (10 female, 33 male) with head and neck cancer was conducted from November 2010 to December 2011. Group A (n = 21) received irsogladine 4 mg/day during radiotherapy and Group B (n = 22) received radiotherapy without irsogladine. Oral Mucositis Weekly Questionnaire─Head and Neck Cancer (OMWQ-HN), Revised Oral Assessment Guide (ROAG), Numeric Rating Scale (NRS) and Common Terminology Criteria for Adverse Events (CTCAE) scores were analyzed by the Mann-Whitney U-test. Quality of life measured by OMWQ-HN gradually declined in both groups. However, the score for Group A was significantly higher than that for Group B at 50 Gy of radiotherapy. A significant increase in ROAG score was observed in both groups, but the score for Group A was significantly lower than that for Group B after 20 Gy. There was no difference in safety score between the two groups as measured by CTCAE ver. 4. In conclusion, it was found that irsogladine reduced oral mucositis lesions associated with radiotherapy in patients with head