A 60-year-old man was admitted to our hospital for fever, loss of appetite, malaise, and skin eruptions on his face and extremities. Chest computed tomography revealed interstitial pneumonia. A diagnosis of anti-CADM-140 antibody–positive classic dermatomyositis (DM) was made based on characteristic skin lesions, muscle symptoms, and elevated CPK levels. This patient was also diagnosed with rapidly progressive interstitial lung disease (RP-ILD). The chest HRCT scan showed a nonspecific interstitial pneumonia pattern on his admission. We initiated steroid pulse therapy, cyclophosphamide pulse therapy, meropenem, and sivelestat at 300 mg/day and direct hemoperfusion with polymyxin B–immobilized fiber (PMX). However, the patient required mechanical ventilation and died of respiratory failure 18 days after admission. On autopsy, lung tissues revealed organizing phases of diffuse alveolar damage. It is known that rapidly progressive interstitial pneumonia with poor prognoses can occur in patients with clinically amyopathic dermatomyositis (CADM). Even classic DM patients have RPIP. Thus we believe that a patient with anti-CADM-140 antibody positive is quite likely to have a poor prognosis, and anti-CADM-140 antibody positive is a poor prognostic factor for classic DM.