1. A fingerprint reading method in which a system comprising a sweep type fingerprint sensor to read a series of partial fingerprint images which represent a fingerprint from a sliding finger, the method comprising: using a processor to carry out the steps of: an image acquisition process of acquiring a partial fingerprint image read by the sweep type fingerprint sensor; a position calculation process of finding position information of the partial fingerprint image in accordance with order of acquisition of the partial fingerprint image when the partial fingerprint image is acquired in the image acquisition process; an image division process of cutting out a unique unit image corresponding to an image region having a predetermined area from the partial fingerprint image on the basis of the position information when the position information is found in the position calculation process; and a feature extraction process of extracting feature information on a feature of the fingerprint itself from the unit image when the unit image is cut out in the image division process, wherein in the position calculation process a decision is made whether the partial fingerprint image is a first partial fingerprint image read from the finger, and when the decision is negative, a relative position of the partial fingerprint image to a preceding image is calculated as position information of the partial fingerprint image.