We report a case of a woman with typical dermatomyositis (DM) with skin manifestations, severe myalgia and muscle weakness complicated by interstitial lung disease (ILD) and pneumomediastinum. Pneumomediastinum persisted despite treatment with immunosuppressive therapy (steroids and cyclosporine). After the test for anti-melanoma differentiation-associated gene 5 (MDA5) antibody came out positive, we doubled the cyclosporine dose and her condition improved. Despite typical clinical features of DM, in cases complicated by pneumomediastinum or steroid resistance, measurement of anti-MDA5 antibody may be useful for immunosuppressant dose titration.