Provided are an image synthesis apparatus, an image synthesis method, and a program thereof, capable of determining a combination of a synthesis candidate image and a background image of which impressions match each other. Plural synthesis candidate images are input, and impression values of the plural synthesis candidate images are determined with respect to plural impression axes. Further, plural background images are input, and impression values of the plural background images are determined with respect to the plural impression axes. A combination of a synthesis candidate image and a background image having a small difference between impression values is determined. The synthesis candidate image and the background image of the determined combination are synthesized to generate a synthetic image.