1. A background replacing apparatus comprising: an image obtaining means for obtaining a plurality of photographic images generated through photography of a common subject under a plurality of photographic conditions; an area discriminating means for discriminating among a subject area, a background area, and a boundary area of each of the plurality of photographic images obtained by the image obtaining means; a mixing ratio determining means for determining a mixing ratio of a subject color to a background color in the boundary area; and a background replacing means for replacing a background of one of the plurality of photographic images obtained by the image obtaining means with a different background in accordance with three areas of the subject area, the background area, and the boundary area, which are discriminated by the area discriminating means, and the mixing ratio determined by the mixing ratio determining means.