1. An arousal-level determining apparatus comprising: a generating unit that generates heartbeat-interval variation data, which indicates changes in heartbeat interval, on the basis of heartbeat signals indicating subject's heartbeats; a calculating unit that applies a band-pass filter, which allows passage of a certain range of frequencies, to each frequency band in the heartbeat-interval variation data while changing the frequency band, and calculates spectral density with respect to each frequency band applied with the band-pass filter; an identifying unit that identifies a feature point corresponding to a spectral density peak in the spectral densities in the frequency bands calculated by the calculating unit; and a determining unit that determines subject's arousal level on the basis of the feature point identified by the identifying unit.