A 52-year-old man was referred to our clinic. One week before his visit, he had complained of left eye pain and double vision. His clinical features were exacerbated. Despite the acute onset, which is atypical of thyroid eye disease (TED), TED was suspected due to the patient's history of Graves' disease (GD). After conducting clinical examinations and orbital magnetic resonance imaging, the patient was diagnosed with idiopathic orbital myositis (IOM), and intravenous glucocorticoid therapy was administered. After treatment, the patient's clinical manifestations dramatically improved. This is a rare case in that the history of GD made it difficult to differentiate IOM from TED.