1. A subject designating device, comprising: a first image generation unit that generates a brightness image expressed based upon a brightness component of a target image and chrominance images expressed based upon chrominance components of the target image; a representative value calculation unit that calculates a representative value for each image of the brightness image and the chrominance images having been generated by the first image generation unit, based upon pixel values indicated at pixels present within a first subject area containing a subject in the each image of the brightness image and the chrominance images; a second image generation unit that creates a differential image each in correspondence to the each image of the brightness image and the chrominance images having been generated by the first image generation unit, by subtracting the representative value calculated by the representative value calculation unit from pixel values indicated at pixels present within a second subject area containing the subject in the each image of the brightness image and the chrominance images; a binarizing unit that creates a binary image by binarizing the differential image having been generated by the second image generation unit; a synthesizing unit that creates a synthetic image by combining the binary image created by the binarizing unit in correspondence to the brightness image and the binary images created by the binarizing unit in correspondence to the chrominance images; a mask extraction unit that extracts a mask constituted with a white pixel cluster from the synthetic image generated by the synthesizing unit; an evaluation value calculation unit that calculates an evaluation value indicating a likelihood of the mask, having been extracted by the mask extraction unit, representing the subject; and a subject designating unit that designates the subject in the target image based upon the evaluation value having been calculated by the evaluation value calculation unit.