1. A biometric information reading device comprising: an image capturing unit that sequentially captures a plurality of partial images about biometric information; an image feature extracting unit that extracts an image feature from each of the partial images and a position of the image feature; a matching relative position detecting unit that determines whether a relative distance between two successive partial images among the partial images is equal to or smaller than a threshold value, based on the image feature of an overlapping region between the two successive partial images; and a partial image output determining unit that determines to output at least two successive partial images as one set, determines not to output a partial image that is input after the partial images as the one set when the matching relative position detecting unit determines that a relative distance of the partial image from one of the partial images as the one set is equal to or smaller than the threshold value, and determines to output at least two successive partial images as another set including a partial image that is input after the partial images as the one set when the matching relative position detecting unit determines that a relative distance of the partial image from one of the partial images as the one set is not equal to or smaller than the threshold value.