We report a 53-year-old male with Cogan's syndrome. He was admitted to our hospital because of a fever of 2-weeks duration, blurred vision for 10 days, hypoacusis, and numbness of the left hand for 3 days. In addition to uveitis, hypoacusis, and aseptic meningitis, multiple mononeuropathy was diagnosed based on a nerve conduction study. Furthermore, positron emission tomography/computed tomography (PET/CT) revealed diffuse aortitis. Accordingly, the patient was diagnosed with Cogan's syndrome. After starting steroid-pulse therapy followed by 1 mg oral prednisolone/kg/day, the uveitis and hypoacusis improved immediately, while the peripheral neuropathy persisted until effectively treated with intravenous gamma globulin therapy. Prompt steroid therapy for Cogan's syndrome based on a diagnosis made using PET/CT prevented progression of the hypoacusis.