1. An image quality evaluation method for evaluating an image quality of a decoded image, being a second image, said decoded image obtained by decoding an encoded image obtained by encoding an original image, being a first image, in a transmission destination, said image quality evaluation method comprising: evaluating the image quality of the second image from a difference with the first image, including from an image frame of one of the images, extracting a representative pixel component value indicative of a pixel component value that represents pixels within the above image frame, and pixel position information indicative of a pixel position in which said representative pixel component value appears as a feature quantity, and by employing the representative pixel component value and the pixel position information each of which is the feature quantity of said image, estimating a difference of said entire second image with said first image based upon a difference between the pixel component value at the pixel position being indicated by said pixel position information, and said representative pixel component value within the image frame of the other image; extracting the representative pixel component value and the pixel position information as a feature quantity of said original image from the image frame of said original image; encoding said extracted feature quantity of said original image; transmitting said encoded feature quantity of said original image to the transmission destination of said encoded image; upon receipt of said encoded feature quantity of said original image, decoding said encoded feature quantity of said original image that has been received; and estimating a difference with said original image as a degree of image quality degradation of said decoded image based upon the representative pixel component value and the pixel position information each of which is the feature quantity of said original image obtained by decoding, and image information of said decoded image.