The trials to detect the cerebral ischemia during the brain surgery have been continued since last three decades. The intraoperative Xenon isotope cerebral blood flow(CBF) measurement and EEG monitoring were proven to be useful techniques for this purpose. But these techniques have several drawbacks and are not easily applicable in most institutions. Authors, therefore, developed a intraoperative cerebral ischemia monitoring system which applied the digital electroencephalography(EEG) and compressed spectral array(CSA) technique. Technical details of our system and the examples of clinical applications are described.