1. An information processor having a CPU comprising: an fingerprint sensor that acquires a plurality of first images obtained by successively reading portions of a relatively moving object; an extrapolation section, executed by the CPU, that extrapolates the plurality of first images to generate a plurality of second images obtained by extending the image areas of the plurality of first images; a calculation section, executed by the CPU that, based on a plurality of images including the first images, the second images or a combination of the first images and the second images, calculates the relative positions between two images among the plurality of images; and an instruction section, executed by the CPU, that issues to the extrapolation section, based on the relative position of a previous first image calculated by the calculation section, an instruction concerning the extrapolation for a first image whose relative position has not been calculated by the calculation section, wherein the instruction section predicts, based on the relative position of the previous first image, the direction of the relative movement at the acquisition time of the first image whose relative position has not been calculated, determines an extrapolation position for the first image whose relative position has not been calculated based on the predicted direction of the relative movement, and notifies the extrapolation section of the determined extrapolation position, and wherein when there exists a third image and fourth image, which are the first images successively acquired by the fingerprint sensor, the instruction section sets an extrapolated portion for the third image in the direction toward the fourth image and sets an extrapolated portion for the fourth image in the direction toward the third image.