1. A biometric information processing apparatus comprising: an image acquisition unit which acquires a plurality of line-shaped images of a fingerprint by means of a sweep-type sensor for identifying individuals; a similarity value calculation unit which calculates a similarity value of a similarity evaluation target by use of an evaluation function for evaluating the similarity between the line-shaped images, the similarity evaluation target containing a first line-shaped image and a second line-shaped image out of the plurality of the line-shaped images, the similarity value representing a degree of similarity between the first line-shaped image and the second line-shaped image; a compression unit which compresses the first line-shaped image and the second line-shaped image when the similarity value is equal to or larger than a predetermined threshold value to thereby generate a new line-shaped image; a generation unit which combines the newly generated line-shaped image with the other line-shaped images to thereby generate an image of the fingerprint; a threshold value determination unit which determines for each line-shaped image the threshold value such that the larger the number of positions where different gradation levels are adjacent each other in a longer direction, the larger the threshold value.