A male infant aged nine months with meningeal irritation and +3.5SD expansion of the circumference of the head was admitted. Brain computed tomography (CT) detected right chronic subdural hematoma and contralateral subdural hygroma. Since the cell count was increased on a cerebrospinal fluid test, acute purulent meningitis was diagnosed. A rapid latex test and culture of cerebrospinal fluid identified Streptococcus pneumoniae (PSSP) as the pathogen. Panipenem/betamiprom (PAPM/BP) was administered at 100 mg/kg/3 times for 14 days and dexamethasone was administered at 0.6 mg/kg/4 times for 4 days, and the patient recovered without sequelae. Acute purulent meningitis complicated by chronic subdural hematoma and subdural hygroma is rare. In this case, acute subdural hygroma may have concomitantly developed with acute purulent meningitis in the presence of the chronic subdural hematoma, and rapid disequilibrium of intracranial pressure may have been the developmental mechanism.