1. A method of discriminating a latent fingerprint residual on an imaging surface of an optical fingerprint input apparatus, which acquires a fingerprint image by a backlight illuminated onto a fingerprint and reflected therefrom, the method comprising the steps of: acquiring an image by illuminating the backlight onto the imaging surface; detecting an existence of a fingerprint from the acquired image; acquiring an image while the backlight is in the “off” state, if a fingerprint has been detected; detecting the existence of a fingerprint from the acquired image; and determining that the fingerprint, if detected, is a latent fingerprint detected due to an external light.