A 63-year-old man revealed a four-month history of muscle weakness of the lower limbs, hypoesthesia of the L5 and S1 area and ischuria. On MRI, the spinal cord was compressed by an encircled mass, which showed hypointensity on T1- and T2-weighted images with gadolinium enhancement at the Th11 to Th12 vertebra. Because of the rapid progression of myelopathy, posterior decompression was performed and idiopathic hypertrophic spinal pachymeningitis (HSP) was finally diagnosed. The patient's neurological signs markedly improved with postoperative corticosteroid treatment. Idiopathic HSP is a clinical emergency and early surgical intervention is essential to prevent irreversible damage to the nervous system.