A 54-year-old woman complaining of muscle weakness and weight loss was admitted to our hospital with suspected polymyositis. Muscle biopsy revealed Langhans-type giant cells and noncaseating granulomas. Therefore, sarcoid myositis was diagnosed. The patient was treated with prednisolone, and the symptoms improved gradually. Generally, sarcoidosis is identified clinically in patients with foggy vision or mediastinal lymphadenopathy, but muscular weakness may be an infrequently observed initial symptom. Sarcoidosis should be considered in the differential diagnosis of polymyositis.