In recent years, dental laser therapy has become increasingly popular and applicable. On the other hand, there is an increasing number of reports of subcutaneous and mediastinal emphysema caused by dental laser therapy. This paper reports a case of mediastinal emphysema caused by CO₂ laser irradiation into the periodontal pocket. The patient was an 80-year-old man. In a dental clinic, for the purpose of curettage in 11 periodontal pockets, the tip of the device was directly inserted and CO₂ laser was continuously irradiated for 5 minutes, and his face and neck swelled significantly. He was referred to our hospital and was immediately admitted with a diagnosis of mediastinal emphysema. CT examination revealed a wide range of aerobic images from his bilateral face to the neck and mediastinum. He was given an antibacterial drug to prevent infection and was discharged on the 5th day because CT showed a decrease in emphysema. He continued to take antibiotics thereafter, and CT confirmed the disappearance of emphysema on the 18th day. It is important to consider the risk of severe emphysema from CO₂ laser irradiation and learn the proper use of laser equipment and proper response to mediastinal emphysema.