We report the effects of double filtration plasma exchange and immunoadsorption therapy which were performed for a case of stiff-man syndrome even though the patient was negative for anti-glutamic acid decarboxylase (GAD) antibody. The patient underwent a course of four double filtration plasma exchanges, which resulted in marked clinical improvement. Painful muscle cramps disappeared and muscle stiffness reduced within a day after the first plasma exchange. The patient's improvement continued, but his condition declined again about ten months after plasma exchange. Immunoadsorption therapy was then performed, and this treatment was also effective.