A 33-year old man was admitted to our hospital because of an abnormal shadow on the chest radiograph, dry cough, and exertional dyspnea. Chest radiograph and high-resolution computed tomography (HRCT) on admission showed ground-glass opacities and bronchiectasis with volume loss in the bilateral dorsal areas. Thoracoscopic lung biopsy specimens showed mainly a pattern of NSIP (nonspecific interstitial pneumonia). We considered this case as hypersensitivity pneumonia or interstitial pneumonia (IP) associated with collagen disease. Oral prednisolone (PSL) was initiated at 55 mg/day (1 mg/kg). However he complained of proximal muscle weakness and pain and difficulty of breathing. He had heart failure due to the myocarditis. We established a diagnosis of IP associated with polymyositis and it was confirmed by his symptoms, muscle biopsy findings and elevation of serum CPK. We considered this case as the myocarditis due to polymyositis.