1. An image data processing method including: a motion data computation process for calculating the motion data between a reference image and a target image; a block comparison process for calculating difference in pixel value between the reference image and the target image for every block matched by the motion data; a blending ratio calculation process for calculating, based on the difference in pixel value, the blending ratio between the reference image and the target image for each block; and an image synthesizing process for creating a synthesized image by synthesizing the reference image and the target image according to the motion data and the blending ratio.