Abstract Pneumomediastinum as a complication of interstitial pneumonia with leakage of air into the mediastinum or subcutaneous tissues is a rare complication of dermatomyositis (DM). Herein we report a case of pneumomediastinum complicating polymyositis (PM), which is usually associated with DM. A 61-year-old man was hospitalized in our department because of deterioration of interstitial pneumonia. Treatment with high-dose corticosteroid and cyclosporin A steadily improved his interstitial pneumonia. Two weeks later, he developed subcutaneous emphysema and chest X-ray showed pneumomediastinum. Both subcutaneous emphysema and pneumomediastinum improved gradually without any additional treatment.