1. A foreign object determination device, comprising: an image acquisition unit for acquiring a plurality of images which capture a same authentication target before and after movement; a finger region identification unit for identifying a finger region including a foreign object as a moving object from the images acquired by the image acquisition unit, by using a color of pixels in the image of the plurality of images representing the same authentication target and a difference of a color of a same pixel in the images of the plurality of images representing the same authentication target before and after movement acquired by the image acquisition unit; a region partitioning unit for partitioning the finger region identified by the finger region identification unit into regions of mutually different colors by using predetermined determination criteria for determining differences in color; and a foreign object determination unit for determining that the foreign object is included in the authentication target when a difference of colors in the regions, size of the regions and/or number of the regions, partitioned by the region partitioning unit, satisfies a predetermined condition.