Most cases of pneumomediastinum are caused by iatrogenic injury during surgery on the cervical region and chest or by tracheostomy. It is also well known that emphysema may occur secondary to dental treatment using an air turbine drill, but there have been few cases of emphysema extending to involve the mediastinum. Presented is a rare case in which subcutaneous emphysema and pneumomediastinum developed asymptomatically, probably due to extraction of a mandibular third molar, and were found incidentally on the day after the dental procedure. To avoid subcutaneous emphysema and pneumomediastinum associated with dental treatment and surgical intraoral procedures such as tooth extraction, air turbine drills should be used only when it is essential.