1. A method for generating a background image, comprising: a preparation step of preparing an unprocessed image which is shot against an arbitrary background, including a material to be trimmed; a color information collection step of collecting color values from a plurality of frames of the unprocessed image at a collection point composed of one or more pixels, thereby obtaining a color information set; and a color setting step of judging from the color information set which color shows the background and which color shows the material to be trimmed, and determining an optimum background-side color value, and wherein the color information collection step and the color setting step are applied to an area where to extract the background from, thereby obtaining a background image into which the background is extracted.