A 31-year-old woman was referred to our hospital and diagnosed as overlap syndrome with systemic lupus erythematosus and dermatomyositis. After completing the fourth cycle of intravenous immunoglobulin therapy, the patient developed acute confusional state with the Glasgow Coma Scale of 7. Considering the lack of response to high dose corticosteroid therapy (methylprednisolone 1 g per day for 3 days), rituximab (500 mg per week) was administered twice. The next day after the administration of the first dose of rituximab, the level of consciousness started to improve and 15 days after rituximab, mental status was fully recovered. The proportion of CD19+ B cells started to decrease within 1 week after the administration of rituximab and remained depleted for 14 weeks. There was also a gradual decrease in serum CD40 and CD80 concentration measured by ELISA up to 4 months. This case suggests the effect of rituximab for the treatment of neuropsychiatric lupus.