1. A face authentication system, comprising: a reflectance image generating unit for generating a blurred image of an input image based on the input image including a face of a person and generating an input reflectance image by separating each corresponding pixel value of the blurred image from each pixel value of the input image; a face authenticating unit for performing face authentication of the face of the person included in the input image by comparing the input reflectance image with a registered reflectance image of a previously registered person; a motion vector calculating unit for calculating a motion vector of the input image or the input reflectance image with respect to a previously registered reference face image; and a position distortion correcting unit for correcting a position distortion of the face of the person included in the input image or the input reflectance image based on the motion vector, wherein the face authenticating unit performs face authentication based on the input reflectance image after a position distortion is corrected by the position distortion correcting unit, and wherein the face authenticating unit moves the input reflectance image, the position distortion of which is corrected by the position distortion correcting unit with respect to the registered reflectance image in unit of a pixel in a two-dimensional direction within a predetermined range, calculates similarity between the input reflectance image and the registered reflectance image in each state of position, and performs face authentication based on the input reflectance image and the registered reflectance image in a state of the highest similarity.