Radiation-induced carotid artery stenosis (RI-CS), a life-threatening condition, can occur after external radiation for head and neck cancer. We here describe a case of asymptomatic RI-CS in a 73-year-old patient treated with chemoradiotherapy and radical neck dissection for a basaloid squamous cell carcinoma of the oral floor. Stenosis of the left carotid artery, diagnosed as RI-CS, showed on an MRI performed 1.5 years after radiotherapy. Blood from the left side of the anterior cerebral artery and the middle anterior artery was flowing to the brain through the anterior and posterior communicating arteries, so no stent surgery or other treatment was necessary. The cancer has not recurred during approximately 5 years of followup after radiotherapy, and the patient has had no adverse effects from the RI-CS since it was diagnosed 3.5 years ago. This case emphasizes the necessity of early scrutiny for RI-CS in patients given radiotherapy for oral cancer.