We describe a 38-year-old man who presented with proximal muscle weakness, myalgia, polyarthralgia, and skin rash and was diagnosed as having dermatomyositis (DM). The patient's symptoms improved with prednisolone therapy. However, myopathy relapsed and pneumomediastinum with subcutaneous emphysema developed. Pneumomediastinum with subcutaneous emphysema rapidly disappeared by the administration of ciclosporin. We reviewed the present case and previously reported cases regarding the clinical characteristics. All of the reported death cases were accompanied by interstitial lung disease (ILD). Although it has been reported that pneumomediastinum in DM can be fatal, the direct cause of patient's death was due to respiratory failure resulting from progressive ILD. Pneumomediastinum without ILD shows a good prognosis.