A 63-year-old male presented with an unusual case of nontraumatic intradiploic arachnoid cyst in the frontotemporal convexity area. Skull radiography showed a circular osteolytic lesion of the right parietal bone. Computed tomography revealed a defect of the inner table, widened diploic space, and thinned outer table. Magnetic resonance imaging showed a cyst containing cerebrospinal fluid. Bone scintigraphy showed no abnormal uptakes. Intraoperative observation confirmed the neuroimaging findings. Histological examination found no abnormal findings in specimens of skull bone or arachnoid membrane. Intradiploic arachnoid cyst is characterized by parasagittal, multiple, welldemarcated osteolytic lesions on radiographs in the elderly. However, the clinical features of this disease remain unclear and diagnosis without an exploratory surgical procedure may not be possible.