A 61-year-old Japanese woman with anemia, submandibular gland swelling, and enlarged lymph nodes was diagnosed with IgG4-related disease (IgG4-RD) by lymph node biopsy. Bone marrow involvement of IgG4-RD was detected by 18F-fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT) and immunohistochemically proven by bone marrow biopsy in this patient. Anemia progressed gradually, and prednisolone treatment was initiated. Anemia and submandibular gland swelling improved soon after prednisolone treatment was initiated. We report a rare case of IgG4-RD involving the bone marrow. FDG-PET/CT and bone marrow biopsy were useful for diagnosis in this case.