The arousal level determination device specifies, as an arousal section, a section in which a subject's arousal level has moved toward arousal in comparison to the subject's arousal level in a section immediately after the subject gets on a vehicle. The arousal level determination device calculates a statistical value of the arousal levels included in the specified arousal section, and sets a threshold value for determining the arousal state of the subject based on the calculated statistical value. The arousal level determination device outputs a warning when the arousal level of the subject drops below the threshold value.