1. A dishonest registration preventive apparatus in a personal authentication system, which collates an image obtained by photographing of a person with a beforehand registered image to authenticate the person on the basis of results of the collation, the apparatus comprising detection means for detecting positions of a predetermined part of the person from a plurality of images acquired by consecutively photographing the predetermined part, calculation means for calculating a variation between a position at this time and a position at the last time for a second position and subsequent positions, which are detected by the detection means, determination means for comparing a variation calculated by the calculation means with a predetermined threshold to determine an image at this time as a registrable image when the variation does not amount to the threshold and to determine the image at this time as an unregistrable image when the variation exceeds the threshold, and storage means for registering and storing images, which are determined by the determination means to be registrable.