A 73-year-old woman was diagnosed as interstitial pneumonia and treated with corticosteroids. Seventeen months later, she developed persistent fever. Considering the possibility of a malignancy, 18fluorine fluorodeoxyglucose-positron emission tomography ([18F]-FDG-PET) was performed, only to detect mild accumulation in the proximal skeletal muscles. She then presented muscle weakness with a myogenic pattern on electromyography. She was consequently diagnosed as polymyositis and then as anti-synthetase syndrome. [18F]- FDG-PET was the key to the diagnosis of myositis in patients without muscle symptoms.