1. An image synthesis processing apparatus that rotates at least one of a background image and a foreground image and then synthesizes the foreground image and the background image, comprising: a first image rotation processing unit configured to rotate the background image by an instructed rotation angle in an opposite direction which is opposite to an instructed rotation direction; a synthesis processing unit configured to synthesize the background image after rotation and an original foreground image, thereby generating a synthesis image; and a second image rotation processing unit configured to rotate the synthesis image by the instructed rotation angle in the instructed rotation direction.