1. A fingerprint authentication system comprising: an image division unit that divides a captured image of an authentication subject to be subjected to fingerprint authentication into a plurality of small regions; a frequency analysis unit that performs a frequency analysis on each of the small regions; a ridge candidate extraction unit that extracts frequency components corresponding to ridge candidates in the small regions from frequency components obtained in the frequency analysis; a continuous ridge candidate group generation unit that compares the ridge candidates between adjacent small regions, and when the ridge candidates satisfy a condition enabling the ridge candidates to be determined continuous, generates a continuous ridge candidate group including the ridge candidates; and a determination unit that determines that an abnormality exists in the image when overlap exists between the image regions corresponding to the continuous ridge candidate group.