1. An authentication apparatus comprising: image processing means for performing predetermined image processes on biometric trait image data sequentially supplied from image pickup means at predetermined intervals, the predetermined image processes including at least a binarization process; detection means for detecting, over time, correlation values between adjoining images of the image data on a time axis and correlation values between adjoining binarized images of binarized image data on a time axis, the binarized image data being generated by the image processing means; and selection means for selecting, when a situation in which both the correlation values of the images and the correlation values of the binarized images are respectively less than a first threshold and a second threshold continues over a predetermined period of time, one of the binarized image data input after the continuation of the situation, to be compared with registration data.