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 first time point when a subject whose heartbeat signal is detected has got on a vehicle whose driver is the subject to a second 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 that is the heartbeat signal 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 statistical values that are an average value and a standard deviation of the subject's arousal levels corresponding to the updated arousal section, wherein the updating includes specifying, 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 has got on the vehicle, wherein the determining includes calculating, when calculating a relationship between a spectral density and a frequency, a spectral density with an autoregressive (AR) model.