PROBLEM TO BE SOLVED: To provide an automatic chroma key synthesizing device and the like for automatically synthesizing an object image which is captured with the background in a substantially single color, on any arbitrary background image.SOLUTION: A source image 61 of an object captured with the background in a substantially single color and a background image 63 to be combined with the object are inputted to an automatic chroma key synthesizing device 1. A Trimap generator 14 uses parameters of a representative color and an allowable range for a background color and an allowable range for an unknown region regarding pixels of the source image 11 to generate a Trimap 65 formed from a foreground region 41, an unknown region 43 and a background region 45. An alpha mask generator 15 and a foreground image generator 16 extract Kg pieces of pixels belonging to the foreground region around a concerned pixel and pixels belonging to the background region at a maximum for pixels in the unknown region 43 of the Tri
