Hypertrophic pachymeningitis is a chronic inflammatory disease, manifesting as a fibrous thickening of the dura mater. Recently, it was reported that IgG4 is related to the etiology of some diseases, such as pancreatitis. Here, we report a case of hypertrophic pachymeningitis related to infiltration of IgG4 positive plasma cells. A 68-year-old man presented at a nearby hospital with a headache and a low-grade fever. A blood test revealed inflammation, as well as elevation of IgG4 level. Magnetic resonance imaging(MRI)revealed diffuse thickening of the dura mater, dominantly in the posterior fossa and cerebellar tentorium. The lesion was enhanced significantly with gadolinium(Gd). An open biopsy was performed to determine pathological diagnosis. Hematoxylin and eosin staining showed infiltration of inflammatory cells, including plasma cells. The infiltrating cells were positive for the IgG4. Post-operatively, the patient was treated with glucocorticoid, and both the inflammation and patient symptoms were improved. In conclusion, IgG4 is related to the etiology of hypertrophic pachymeningitis and glucocorticoid therapy is effective for this disease.