Immunoglobulin (Ig)-G4-related disease is a multi-organ disease that may affect the lung. We herein describe a patient with IgG4-related lung disease (IgG4-RLD) who was radiologically and pathologically diagnosed with organizing pneumonia. He was successfully treated with a combination of prednisolone (PSL) and azathioprine (AZA), and his clinical course has been uneventful since tapering off PSL. This is a rare case of IgG4-RLD manifesting as organizing pneumonia, and, to our knowledge, this is also the first case showing the effectiveness of AZA in treating IgG4-RLD.