1. An arousal-level determining apparatus comprising: a memory; and a processor coupled to the memory executes a process configured to: receive heartbeat signals for a subject; generate heartbeat-interval variation data, which indicates changes in heartbeat intervals, on a basis of the heartbeat signals indicating the subject's heartbeats; apply a plurality of times a band-pass filter, which allows passage of a frequency band of a certain range of frequencies, to the heartbeat-interval variation data, the frequency band being changed for each of the plurality of times, and calculate spectral densities with respect to each frequency band, by overlapping a range of frequencies within a first frequency band of the band-pass filter with a second frequency band of the band-pass filter that is nearest to the first frequency band, to detect peaks respectively of the calculated spectral densities; identify as a feature point a peak in the calculated spectral densities based on the detected peaks; and determine the arousal level of the subject on a basis of a spectral density and a frequency of the identified feature point, the arousal level inversely indicating a degree of sleepiness of the subject.