The patient was a 70-year-old man with idiopathic orbital inflammation (IOI) that appeared on the severely affected side of preceding myasthenia gravis (MG). The patient was diagnosed with MG 5 years prior to the onset of IOI. When IOI was diagnosed, an edrophonium test was negative. IOI was considered because he complained of left orbital pain, eyelid swelling, and cerebral MRI exhibited the enhanced lesions along the left orbital periosteum. A biopsy specimen revealed pathological findings compatible with IOI. The administration of corticosteroids was effective for improving the ocular symptoms. IOI should be considered when ocular symptoms deteriorated with soft tissue swelling/pain in MG patients.