The patient was a 75-year-old female who presented with rheumatoid arthritis and bronchial asthma. She was admitted following a 10-kg weight loss over the course of 6 months and a slight degree of dysphasia in May 2011. Hypoxia was observed at the time of admission, and she developed acute exacerbation 7 days after admission. A chest X-ray and chest CT revealed no parenchymal lung disease. Slight weakness was found in the muscles of her limbs. She complained of pain when the muscle of her proximal limb was grasped. An electromyography was showing a myogenic pattern. CK and aldolase were elevated mildly. The diagnosis was polymyositis. She was intubated and mechanical ventilation was initiated. Steroid and intravenous gamma globulin was initiated, and her symptoms were gradually improved. We encountered a rare case of polymyositis diagnosed as a result of respiratory failure. This case illustrates the need to consider polymyositis as well as other neuromuscular diseases when evaluating individuals developing respiratory failure of unknown origin.