We present a case of hypoxia which occurred during the onset of general anesthesia in a small hospital. It was found that one of the oxygen tank which formed the central pipeline gas supply had been erroneously replaced by a nitrogen tank. Lack of strict observance of Compressed Gas Supply Standards by the gas supplier and the hospital personnel allowed it. We also emphasize that the oxygen analyzer should be counted as an essential monitor in every anesthesia. Oxygen analyzer detects the supply of intraoperative hypoxic gas admixture promptly and effectively.