We report a case in which an undernourished female patient underwent drainage for gingivitis, and subsequently suffered S. intermedius-induced subdural abscess, meningitis and transverse sinus thrombosis. A few days after drainage, she had a fever of 39°C and became lethargic with non-fluent aphasia. Cerebrospinal fluid revealed pleocytosis of 1269/μl, protein 222 mg/dl (normal 15-45mg/dl), glucose 33 mg/dl (ratio to blood glucose: 0.37). The diffusion-weighted MRI brain showed an area of abnormally high signal along the left brain surface. In the magnetic resonance venography, the left transverse sinus was not well delineated. After treatment with antibiotics (meropenem, vancomycin) and heparin, craniotomy was performed to remove the abscess. Culture of the abscess tissue detected S. intermedius. After surgery she gradually improved. To our knowledge, this is the first report of subdural abscess and transverse sinus thrombosis caused by S. intermedius occurring as a result of drainage treatment for gingivitis.