Objective: Although much is known about acute carbon monoxide (CO) poisoning, little is known about chronic CO poisoning. Chronic CO poisoning is often diagnosed based on the patient’s living environment and medical history. Herein, we report the case of an older patient who presented with repeated unconsciousness due to chronic CO poisoning.Case presentation: A 90-year-old man was brought to the emergency department after being found at home with a disturbance of consciousness. Arterial blood gas measurements in room air revealed a carboxyhemoglobin level of 18.0%. Impaired consciousness was caused by chronic CO poisoning. The patient received high-flow oxygen therapy, which promptly improved his condition. According to his family, briquette kotatsu was the cause of chronic CO poisoning.Conclusion: Although high-flow oxygen therapy has been said to be less effective than hyperbaric oxygen therapy in CO poisoning treatment, recent studies have demonstrated that high-flow oxygen has similar effects and benefits. Thus, in institutions that do not have hyperbaric oxygen, high-flow oxygen may be sufficient to treat patients with CO poisoning, as seen in the present case. It should be noted that briquette kotatsu can lead to CO poisoning. This case highlights the need for clinicians to consider patients’ living conditions.