1. A method of discriminating a latent fingerprint residual from that of a biomass on an imaging surface of an optical fingerprint input apparatus, which acquires a fingerprint image by a backlight illuminated onto a fingerprint and reflected there from, the method comprising the steps of: acquiring an image while switching on and off the backlight onto the imaging surface; detecting an existence of a fingerprint with respect to the image part acquired under an “on” state of the backlight; detecting the existence of a fingerprint with respect to the image part acquired under an “off” state of the backlight; and determining that the fingerprint, if detected from both image parts acquired under the “on” state and “off” states of the backlight, is a latent fingerprint detected due to an external light.