A 74 year-old man presented with complaints of dysuria and miction pain. Since the prostate volume was 43.5 ml, the patient was scheduled for surgical treatment of benign prostatic hyperplasia. However, prostate cancer was suspected from the magnetic resonance imaging findings and a prostate biopsy was performed. No malignant findings were observed in the pathological results, but numerous plasma cells stained positive for IgG4. Abdominal computed tomography showed pancreatic head enlargement with surrounding inflammatory changes and elevated serum IgG4 was also observed. The patient was diagnosed with IgG4-related disease (pancreatitis/prostatitis). Dysuria improved with induction of 30 mg prednisolone. The patient no longer needed to take the α1 blocker and 5α reductase inhibitor. The international prostate symptom score and urine flow measurement indicated that the patient remained in good condition at 18 months since the start of treatment.