Purpose: In an irradiated jaw, the level of cellular activation in bone decreases, and so bacterial infection is likely to cause osteomyelitis in such cases. Herein, we report a case in which conservative treatment for mandibular osteomyelitis due to periapical periodontitis of tooth 46, which developed in a patient with a history of radiation therapy for pharyngeal cancer, led to good progress.<br> Case description: The patient was a 58-year-old male. Chemoradiation therapy was performed from November 2006 to January 2007. In early May 2010, the patient suffered spontaneous pain on the right side of the mandible. At a periodical examination in the Department of Otorhinolaryngology in the same period, ¹⁸F-fluoro-deoxyglucose positron emission tomography-computed tomography (FDG-PET/CT) revealed high uptake of FDG on the right side of the mandible. The subject was diagnosed with radio-induced mandibular osteomyelitis due to periapical periodontitis of tooth 46 at the Department of Periodontics and Endodontics and Department of Oral and Maxillofacial Surgery of Okayama University Hospital. A dose of antibiotics and painkillers (non-steroid anti-inflammatory drugs) was commenced at the Department of Oral and Maxillofacial Surgery, and root canal treatment for tooth 46 was continued at the Department of Periodontics and Endodontics.<br> Treatment progress: Despite continued root canal treatment for tooth 46, spontaneous pain on the right side of the mandible persisted. Furthermore, the region of percussion pain according to Yumikura symptoms spread forward from tooth 46. The patient had a history of radiation therapy to the mandible, hence surgery of the mandible was contraindicated. Accordingly, instead of surgery, the root canals of all teeth that were present in the area of edematous change in the bone marrow determined by magnetic resonance (MR) imaging, were opened. Concurrently, in addition to discontinuation of anti-cancer medication at the Department of Otorhinolaryngology, a dose of antibiotics was continued at the Department of Oral and Maxillofacial Surgery. Moreover, by keeping those teeth and the affected area rested, the acute symptoms of osteomyelitis improved. The mandible has since remained in a good condition.<br> Conclusion: For radio-induced mandibular osteomyelitis due to periapical periodontitis of tooth 46, combined conservative treatment by root canal treatment and a dose of antibiotics led to good progress. The removal of oral infection under collaborative medical and dental care for patients scheduled for radiation to the jaw is important for preventing radio-induced mandibular osteomyelitis.