An 84-year-old man was referred to our out-patient clinic with an elongated mass localized to the retrosternal area that was incidentally identified by computed tomography. On 18F-fluorodeoxyglucose-positron emission tomography, this lesion showed intense tracer uptake. Thus, a surgical biopsy under thoracoscopy was performed. Histological examination revealed dense fibrous tissue associated with inflammatory cell infiltration. The immunoglobulin (Ig) G4/IgG plasma cell ratio was over 90%. Serum IgG4 levels were normal. According to the Umehara criteria for IgG4-related disease, a final diagnosis of a "possible" IgG4-related fibrosing mediastinitis was made. Oral glucocorticoid treatment with 30 mg/day prednisolone reduced the mass.