1. A non-transitory computer-readable recording medium storing an arousal level determination program that causes a computer to execute a process comprising: first setting an arousal section including from a time point when a subject has got on a vehicle to a time point when a predetermined time has elapsed thereafter; determining whether an arousal level of the subject has moved toward arousal in a section which is temporally later than the arousal section, based on a biosignal detected from the subject; when the arousal level of the subject has moved toward arousal in the section which is temporally later than the arousal section, updating the arousal section with a section in which the arousal level of the subject has moved toward arousal; and second setting a threshold value for determining whether the subject is in an arousal state, based on a statistical value of the subject's arousal levels corresponding to the updated arousal section.