1. A subject extracting method, comprising: processing a pre-processed image represented in an RGB color space to generate a luminance image including luminance components and a chrominance image including chrominance components; generating binary mask images for extracting a subject by binarizing each of the luminance image and the chrominance image with a predefined threshold value; determining one binary mask image selected using a predetermined evaluation value from a binary mask image of the luminance image and a binary mask image of the chrominance image as the binary mask image for extracting the subject; and based on the pre-processed image and the determined binary mask image, generating a subject image in which only the subject included in the pre-processed image is extracted by eliminating a background region from the pre-processed image.