This case involves a 69-year-old man whose main complaint was a right chest pain. Indicated with right pleural effusion and mediastinal and hilar lymphadenopathy by a neighboring doctor, he was admitted to our hospital. From elevated serum IgG4, interleukin (IL)-6 levels, IgG4-related disease (IgG4-RD), or multicentric Castleman's disease (MCD) were suspected. The examination of pleural effusion revealed IgG4 positive plasma cells and eosinophils. The right pleural and the mediastinal lymph node biopsy specimens revealed IgG4 positive plasma cell infiltrations. Because of the lack of anemia, thrombocytosis, and hypoalbuminemia, we concluded that the case was IgG4-RD. Generally, the histological findings of IgG4-RD and MCD were similar, and it is necessary to take these clinical findings into account. We should keep in mind the possibility of MCD in an out-patient clinic.