(1) In the electroencephalogram of cerebral arachnoiditis, no spike was observed. As abnormal waves, the slow wave of 6-7 cycles was only seen. For the determination of the lesion, the size of the amplitude of α-wave was evalutaed. (2) Arachnoiditis optochiasmatis, stenosis of the Foramen Magendie caused by arachnoid adhesion and arachnoidites of the cisterna magna were not able to be diagnosed by this method. (3) In all cases of arachnoiditis of the cisterna lateralis and arachnoiditis overlying cerebral hemisphere, localization of foci was possible.