An apparatus for monitoring an anesthetic depth of a living subject, including at least two devices of (a) an arousal-index determining device which determines an arousal index indicating a degree of arousal of the subject, (b) a pain-index determining device which determines a pain index indicating a degree of pain of the subject, and (c) a muscular-relaxation-index determining device which determines a muscular-relaxation index indicating a degree of muscular relaxation of the subject, and an anesthetic-depth determining device for determining, based on at least two indexes determined by the at least two devices, an anesthetic depth of the subject according to a predetermined relationship between anesthetic depth and the at least two indexes.