We report a 40-year-old woman with primary Sjögren's syndrome (SjS) with slowly progressive CNS disease. At age 38, she noticed spasticity and very gradual onset of monoparesis in the left leg. She hardly walked by herself at age 40. On admission, neurological examination revealed mild slurred speech, vertical nystagmus, spasticity in the four extremities, spastic monoparesis of the left leg, exaggerated jaw jerk, hyperreflexia in all limbs except right biceps and brachioradialis reflex, and positive bilateral Hoffmann reflexes and Babinski signs. Laboratory examinations disclosed positive anti-nuclear antibody (speckled type) and anti-SS-A/Ro antibody (64x). CSF examination revealed cell count 8/mm3, protein 42 mg/dl, 4 bands of oligoclonal band and the elevation of IgG index. MR-imaging presented multiple plaque-like lesions in white matter of cerebrum and brainstem, which did not show gadolinium enhancement. Additionally she complained of dry eyes. Lacrimal and salivary secretion tests showed hyposecretion of tears and hyposialosis. The biopsied specimen of labial minor salivary gland revealed the destruction of the ducts and periductal lymphocytic infiltrations. The diagnosis of primary SjS was confirmed. We herein report a rare case of primary SjS with CNS disease mimicking chronic progressive multiple sclerosis (MS), and discuss a difficulty in differentiating CNS disease of SjS from MS.