A red LED which emits red light and a green LED which emits green light are provided as light sources to emit detection beams onto a fingerprint collation surface. With a finger tip pressed against the fingerprint collation surface, only the red LED is enabled to emit light and reflected light from that surface at this time is input to an image pickup unit, thereby providing a first image. Likewise, only the green LED is enabled to emit light, thereby providing a second image. A difference between intensity levels of reflected light according to the undulations of the fingerprint is determined as a first differential contrast for the first image or second differential contrast for the second image. When the absolute value of a difference between the two differential contrasts is greater than a threshold value, the finger tip is determined as a genuine living finger tip.