A 67-year-old woman with fever and cough was diagnosed with eosinophilic pneumonia because of eosinophilia and increased eosinophil levels in the bronchoalveolar lavage fluid and transbronchial biopsy lung specimens. However, prednisolone therapy at a previous hospital was ineffective. Histological findings from thoracoscopic lung and lymph node biopsies were consistent with multicentric Castleman's disease (MCD). Since specimens also showed prominent eosinophil and IgG4-positive plasma cell infiltration, it was difficult to distinguish IgG4-related disease (IgG4-RD) from MCD. Administration of prednisolone plus tocilizumab improved the symptoms and lung lesions, and prednisolone administration was successfully reduced and then terminated. The present case highlights the difficulty in diagnosing MCD and IgG4-RD, and suggests that combined administration of tocilizumab and prednisolone might be effective in such a case.