We present an extremely rare case of basal meningoencephalocele that protruded into the left pterygoid fossa from the middle cranial fossa. After a 30-year-old woman experienced a generalized tonic-clonic seizure, computed tomography (CT) scanning showed a large bone defect of the left middle cranial base. A mass lesion with low signal intensity on T1- and high signal intensity on T2-weighted magnetic resonance imaging (MRI) was visualized in the left pterygoid fossa. A biopsy was performed through the transantral approach. Because cerebrospinal fluid (CSF) was observed to flow from the mass lesion, it was diagnosed as a meningoencephalocele. CT showed bone defects in the skull base and a well-circumscribed expansile mass lesion. MRI demonstrated that the brain parenchyma extended into a CSF-filled meningocele. These findings demonstrate that CT and MRI provide adequate information for the diagnosis of meningoencephaloceles.